Make sure you are turning up the correct volume control. On the Android platform, there are several different volume settings for the various different types of audio content, such as call volume, media volume, ringer, etc. If you attempt to turn up the volume while you are in the call then it should be adjusting the correct volume, but if you turn it up while you are on say the home screen, then it will just turn up the ringer volume and leave the in call volume the same.

To be sure, you can download an application like Quick Settings from the market, it allows you to have a GUI that will allow you to change the specific volume settings from one screen.

 

Im having the same problem, when I listen to music/video recordings on my backflip,
its low. That is just the phones settings... I tried seeing if I could fix it, but there was no way of making it a bit louder.

 

Possible solution