there is one way to improve u r music in nokia 6300 and also in F.M just download some important music softwares just visit to this sites http://www.getjar.com/software/Nokia/6300/Search/music+player And u can increase u r FM and music just download and install softwares.
For FM, there's really no way because the DJ's have control over what they're broadcasting, not to mention the type they're broadcasting.
For music, I recommend converting your MP3's (or any other music file) into it's highest quality (also bigger in size). Try converting it into a 480000 sample rate with 320kbps bitrate.
Also, I think your music problem isn't really the music file but the output (the speakers or headphones or headset which the sound is coming from). Try using a better speaker or headset or earphones. Use Bose or JBL, also try Skullcandy headsets.