how to stop 3rd party video player from opening mp3 files When I install MoboPlayer it becomes the default mp3 player for Gmail.  This is a problem because it can't actually play mp3 files associated with Gmail (it can't find the file location).  

All the advice I see on the web is to go into manage applications and clear the defaults, but the App Info says MoboPlayer has "No defaults set" and Clear Defaults is grayed out.  I don't see any way to change the default file associations within MoboPlayer. I tried Default App Manager Lite and it also didn't see any defaults associated with MoboPlayer.

Questions:
a. Is there any way to stop MoboPlayer from opening mp3s.
b. Why is it gmail opening mp3s in MoboPlayer anyway since apparently it's not set to open anything by default?

I like MoboPlayer, but I don't have a huge problem ditching it.  I'm really more curious about b.  Why is gmail using it to open mp3 files when it's not the default???

I have a Nexus 4 with Android 4.2.1.
This is most likely caused by MoboPlayer being the only installed app having itself registered for MP3 capabilities. If you install any app which also announces this to the system (e.g. a second player), you should be prompted which one to use. Of course this would only offer MoboPlayer and that second app as choices.

Alternatively, you might take a look at Open With. I did not test it, but according to its description it should intercept here and offer you to select the target app you want to use. Not sure if it only integrates with the "Share" menu -- but certainly worth a try.