It would help if you state what computer OS you are using (Mac or Windows and what version).
I, personally, don't find the HTC Sync Manager to be very useful software. It gets in the way more than it helps.
Assuming you have a Windows 7 or 8 computer, you should be able to simply plug the phone into one of the computer's USB ports using the cable that came with the phone, and the phone will show up on your desktop as a storage device that you can copy files to and from. Then, just go into the music folder on the phone storage and copy the music files from it to your computer's music folder. All done.