There is a cab to install the messenger app (Oz) from the stock Sprint rom.
Here is a
thread with various carrier cabs posted.
Depending on whether you go with a Manila 2.1 or 2.5 rom you might have MMS issues. Manila 2.1 roms MMS works for the most part (usually after installing carrier provisioning cabs and MMS fix cabs), where as with Manila 2.5 you usually have to disable the HTC messaging app or some other kind of work around. But the actual fix will be rom specific and usually posted in the FAQ section for that rom.
Here is
a thread with a good Sprint provisioning cab that allows SprintTV to work.
I also agree with the other posters, Nitrogen player is a free mp3 app that's really good. It's light on memory usage, very finger friendly, I am using version 1.2 and it works great. You can try it out on your stock rom to see how you like it.
(
Download Nitrogen Player 1.2 here)