When you say that you can't play music, could you please elaborate on what exactly is happening?
Have you checked your music player media volume? (It's separate from the notifications and ringer volumes)
Does the music player appear as if the music file playing?
Are there any error messages popping up?
Is the music player app force closing on you?
Even though the Droid is smart enough to make the connection, I'd still be curious as to where you've located the music files.
Also, what format are they in? What I did when I got mine was mount the device to my laptop and create various folders in /sdcard that the device would later automatically associate with the given application: Music - Throw all your .mp3's, .ogg's, wav's, etc... in here Alarms - So that you can select custom tones in the Alarm app instead of using defaults. Pictures - Just for organizational purposes. The Droid will combine any pictures and video you've captured into the Camera folder, so I manually split them up. Once the Camera folder is empty it will disappear from your gallery. Video - Same
I managed to download only a portion of my MP3 files onto the scan disk (even though I have over 300); however, now that I click on the MUSIC SPEAKER AND ALSO THE MP3 DOWNLOAD: I see the songs, but I get a display error: Sorry the player does not support this type of audio.....?