Bad news
<took me too long, some of this was mentioned above>
Unfortunately there are a few issues involved that are not easily circumvented. Chances are your OS X partition is HFS+ and the windows partition is either FAT32 or NTFS. Here's a breakdown of which OS can access what (limited to the scope of this post) without special tools:
OSX:
FAT32 (Read/Write)
NTFS(Read)
HFS+ (Read/Write/Boot)
Windows (Again, limited to the scope, so assuming XP):
FAT32 (Read/Write/Boot)
NTFS (Read/Write/Boot)
HFS+ (none)
With only two partitions, it's impossible to have a bootable partition for OS X and a bootable partition for Windows and have them both readable (much less writable) while running either system.
The preferred solution IMO is a third partition that is FAT32 that both systems can read and write, for passing things back and forth. Your music could live there. There are things you can't store there, like OS X programs (because of resource forks, subject of an entirely different thread), but you shouldn't need to be sharing those between environments.
The next question is how to create this partition. As far as I know, without reinstalling Windows, you can't. I would recommend a combination of using carbon copy cloner to copy your current windows partition, then deleting the windows partition using disk utility. THEN you can divide the empty space left in its wake into 2 partitions, one for your windows installation, one for your shared space. You can then use carbon copy cloner to restore the windows partition to the appropriate place.
I have not tried this method. If you're not prepared to reinstall windows from scratch, don't try this.
As an aside, i'm not sure what your need for windows is (AutoCAD? Games? Web development?) but parallels or VMWare Fusion might suit you if you don't need direct hardware access in windows. Then you could listen to music in OS X and work in windows in the virtualized environment.