As to the audio/video issue,
I followed your advice and ran a test by creating two posts. (See the latest two posts on my website.)
One is with a .mp3 audio file embedded, using the wordpress built-in audio post format, and the other with a .flv video file, using the video post format. (See the attached images 3&4.) Both of these two media can be played normally in the Media page of the backend panel. (See the attached images 1&2.)
And now the .mp3 audio file can only be played on Firefox 27.0.1; Internet Explorer 11.0.9600.16518 can’t play it correctly. Confusingly the .flv can be played on neither Firefox nor IE.
Would you please help to check again? I deeply appreciated your kindness. 🙂