From MozillaZine Knowledge Base
Sometimes the audio or video content embedded in a web page doesn't play in Firefox or SeaMonkey but it works in another browser like Internet Explorer. This usually happens either because you are missing a plugin or because the web page is coded specifically for Internet Explorer. This article will help you solve these and other problems with audio and video content on web pages.
Except for certain open media content, current Mozilla browsers require the help of media players and browser plugins to play audio and video. (See bug 799318 for plans to support playback of additional media formats in future versions of Firefox, using the HTML5 video and audio elements.)
You may already have the correct media players installed that can play audio and video files, but this does not guarantee that you have the plugins needed when the audio or video is embedded inside the web page itself. These articles include information about some common browser plugins and how to set them up:
Missing plugin alerts
Each plugin contains a list of file extensions and MIME types, (also called Internet media types) which it will play. Unlike Internet Explorer, Mozilla browsers will only use a plugin to play the media content if the plugin lists that MIME type; otherwise, you will get a "missing plugin alert". You can see the MIME types and file extensions that are handled by your installed plugins in the about:plugins list.
- The Windows Media Player plugin lists only the Windows Media formats (.asf, .asx, .wm, .wma, .wax, .wmv, and .wvx), and WMP provides no way of changing that list.
- The official RealPlayer 10.5 plugin only lists the .rpm (Real Media Playlist) format, so instances of other embedded Real media files (.rm, .ram, .ra) may not work.
- In Windows XP and below, the QuickTime plugin lists the Quicktime formats and a few others by default, but QuickTime can be configured to handle other media formats. The solution is to go to your QuickTime Preferences and change the QuickTime browser settings by selecting the MIME types you want QuickTime to handle, such as those for MIDI, WAVE, MPEG and MP3 audio formats. Note: In Windows 7/Vista, the QuickTime browser plugin is already configured to handle all supported MIME types (as shown in about:plugins) and cannot be reconfigured.
If you have more than one plugin installed that can handle the same media type, this can cause a conflict that prevents some embedded media from playing. For example, you may need to disable the optional VLC Media Player "Mozilla" plugin in order to play embedded QuickTime or Windows Media content, since the VLC plugin is also set to handle those MIME types but may fail.   Also, the "Windows Media Player Plug-in Dynamic Link Library" plugin included in Windows XP (npdsplay.dll) may interfere with the newer Microsoft Windows Media Player Firefox Plugin (np-mswmp).  You may be able to fix the problem by disabling the conflicting plugin in the Add-ons Manager:
- On the menu bar, click on the Tools menu (or in Firefox 4 and above, click on the Firefox button) and then click Add-ons (Add-on Manager in SeaMonkey).
- In the Add-ons Manager, select the Plugins panel.
- Select the suspected conflicting plugin (e.g., the VLC plugin) and click the Disable button.
- Test the problem media page.
To re-enable a disabled plugin, follow the same steps but click the Enable button.
If you don't have the needed plugin or if the web page is coded specifically for Internet Explorer, you can install an add-on as a workaround:
- The MediaPlayerConnectivity extension (available for both Firefox and SeaMonkey) will launch embedded media on a web page using an external media player, without the need for plugins. 
- The Greasemonkey extension with the IE Media Mimic script will make embedded audio and video play with the Windows Media Player plugin. The script also modifies how they are embedded, to make them behave in Mozilla browsers more like how they would behave in Internet Explorer.
- Some web pages deny you access entirely unless you use Internet Explorer, often because they haven't bothered to test on other browsers, or because they think it only works in Internet Explorer. You can use the User Agent Switcher extension to trick them into thinking you're using IE, but it doesn't mean that it will then work.
- You can install an add-on to open an instance of Internet Explorer for web pages that will not work any other way. Some extensions that will do this are IE Tab 2 IE View and IE View Lite.
Direct links to media files
When a browser plugin is installed that can handle a media type, it will be used to open direct links to media files (links that end in file extensions such as .asx or .mp3) in addition to embedded content on web pages. If you are having problems with direct links to media files or if you would rather not open such links in a plugin, you can change your (Helper) Application settings to use another application to open those types of files, instead of using the plugin. For example, you can change .mp3 file types to open in an installed media player such as the VLC Media Player application, Windows Media Player application, or WinAmp, instead of in the QuickTime browser plugin. Note that certain file extensions, including .mp3, include multiple content type entries, one for each MIME type associated with that type of file. See File types and download actions and Opening files using plugins for more information.
- Some sites may require you to allow popup windows. Allow the site as an exception in your popup blocker settings.
- Try clearing the cache.
- Some sites may require you to allow cookies or that you disable ad-blocking  so try the following:
- Make sure that cookies are enabled in your privacy settings and that the site is not being blocked in the exceptions list. Also try removing cookies for the problem sites to allow new cookies to be set.
- Temporarily disable any adblocking software or extensions such as Adblock or Adblock Plus. If the site works with ad-blocking disabled you may want to "whitelist" that specific site in your adblocking filter.
- If you use a Windows hosts file you can temporarily disable it by renaming it Xhosts, or you can edit the hosts file to remove specific entries such as ad.doubleclick.net that can cause videos on certain sites not to work . You may need to flush the DNS cache  ("Start -> Run -> ipconfig /flushdns") for the hosts file change to take effect.
- Problematic extensions can interfere with audio or video content, including the RealPlayer Browser Record Plug-in extension (installed with RealPlayer) and NoScript. Disable your extensions (restart in Firefox Safe Mode to disable all extensions) or test the problem site using a newly-created profile.
No Sound in Firefox
- Windows Vista has a per-application Volume Mixer that sometimes mutes or turns down the volume in Firefox or another application.  Go to a site that should have sound (see Testing plugins). With your Mozilla application still open, click the speaker icon in the Windows task bar, click "Mixer" and make sure the sound isn't muted and that the volume slider isn't at the bottom.   To restore a missing speaker icon, right-click on clock or tray -> Properties -> Volume: On