How to increase the volume in an m4a file?
Solution 1:
I'd use mp3gain or AACGAIN for that. The former's GUI works with AACGAIN too, but AACGAIN is the app for m4as.
It'll let you batch modify the volume of files to a certain level without loss of quality.
Solution 2:
You need to look at "Auto Volume Levelling". This is done on your PC/MAC.
This page has instructions for iTunes as well as other music apps.
Though if you've got a background hiss already, this could get louder too. You'd need to look at some more serious filtering software for that.