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If your files do not play properly, check the following points:

So what file formats are required?

Only mp3 for audio, and m4v for video. When Ogg versions are missing, JavaScript is used to force Forefox to fall back to Flash. To fully support HTML5 audio and video and bypass JavaScript, include the Ogg versions. All other audio formats are optional. To support migration from legacy video files, it is also possible to use this plugin for playback of FLV files only.

The final HTML code looks fine but doesn't play in Firefox/Safari

The server needs to be configured to serve the video files with the proper MIME type. (Setting the type through HTML code alone is not sufficient.) On a Linux host, you can add the following lines to the .htaccess file:

AddType video/ogg ogg ogv

AddType video/mp4 mp4

Are there limitations to the extensions?

The plugin requires specific file formats and extensions. For Ogg formats, the extension has to be .ogv or .ogg for video, and .oga or .ogg for audio. H.264 format needs the extension .m4v, and MPEG-4 audio m4a. Follow the instructions under Description for the best results. If you decide not to create two versions of video (using e.g. ffmpeg2theora), and you provide only the H.264 file, the plugin will detect this and will use JavaScript to play the file in Firefox using the fallback player.

The plugin does not recognize the location of my files

The current version of the plugin will fail if you link to the audio or video using the attachment ID. If you store the files on a different server or outside of your wp-content directory, use the format option to manually specify the available file formats; this bypasses the autodetection that requires the files in the content directory of WordPress. On some configurations, you need to specify the location using a server path (e.g. [audio src="/wp-content/uploads/mysong"]); this can be outside of wp-content but if it is, specify the 'format' option.

Can I change the look or behavior of the fallback players?

For audio, you can. Refer to the available options and add lines to the file degradable-html5-audio-and-video.php under the heading Format the player by inserting lines here.

What if the plugin conflicts with another plugin?

There is a known conflict with Viper's Video Quicktags. If you use that, download a modified version of Degradable HTML5 plugin from my website. This uses syntax [html5audio ...] and [html5video ...] to prevent the conflict. Audio Player might also not work but you can use Degradable HTML5 plugin to play the audio files instead (it uses the same Flash player and will give the users native HTML5 playback on supported browsers). If you experience problems, try deactivating plugins to determine if there is a conflict.