yes, it can play music, it can play MP3 and MP4, I'm trying to put in MP4's. I have a 2GB micro sd with a couple songs on it, and it plays the songs and videos that are on the card.
You said "I'm trying to put in MP4s" - so you're talking about MP4 VIDEO files instead of actual MP3 AUDIO files. Is that correct?
MP4 files are video and not audio/music files, so they won't appear under the "music" section of your player.
Plus, a Rockchip chipset player will not play MP4 files, only converted ones to .AVI with specific height x width resolution.
What screen size is your player?