If you like Pocket music, or any other simple MP3 player I think you will really like Microfi's Nitrogen open source MP3 Player. Since my phone is my stereo system on my motorcycle I really like easy to use, finger freindly media players and this really fits the bill.
To get it to work on the Treo Pro, I took the base qVGA basic skin and reworked it to fit the 320x320 resolutions. I did not add the wheel as I didn't see that the wheel really added any additional functionality. I instead opted to make the buttons a bit oversized then they appear on the screen. I figured that would help with fat fingers. You can see the actual touch area if you hold down a screen button.
If you would like to try this FREE player
Download the Player Here
If you would like to use my skin your can
Download My Skin Here
If you would like to create your own skin you can
Download a Skin Maker Here