@Gega
I am no expert in this field, but I am interested in making game music, although this is a somewhat distant goal.
But I have used MOD / XM / S3M type files in my projects previously, and did stumble upon Milky Tracker.
For making such music it is well suited, and I hear the MilkyTracker community is very helpful.
I have also used Audacity briefly in the past for simple editing of game sound effects, so it too comes to mind.
Not sure if they suit your requirements, give them a Google and see? ;)

Audacity® is free, open source software for recording and editing sounds.
MilkyTracker is an open source, multi-platform music application for creating .MOD and .XM module files.

Try LMMS - Linux MultiMedia Studio. Despite its name, it's been ported to Windows as well. They say it's not far away from FL in terms of interface, so chances might be good you'll find it interesting.

Depends on what you mostly want to do. When you say "free" there isnt that much to offer in audio editing. But like the last post said LMMS would be perfectly fine if you have used FL studios for awhile. Rosegarden or Ardour is fine too, however I think you are on windows :/. But if you want to try an audio software thats not free but it has a timeless evaluation version. Go download and try Reaper