Restart the phone. You can only play what's in the media database wen you use the player app, and the database doesn't get rebuilt when you copy files, only when you start or restart the phone or do something media-related, like record something or take a picture (or run
media.Re.Scan to force a rescan, which is how I do it).
(If the music app had to scan everything to find all the music files every time you ran it, you wouldn't be happy - scanning takes time.)