Most of my music are missing during USB connection??

[Aelvir]

I recently added an album and when I did it had showed all my music I had on there ini the Music folder properly during usb connection. But now, I have all my music ON THE SD Card when I open the music player. But when I plug it into my computer via USB, it only shows very little of the music I have. Even the album I added most recently (being the Undertale Soundtrack I bought) doesn't show. Same goes for In The End by Linkin Park and the Final Fantasy songs I had on iTunes. Before I added the Undertale Soundtrack I had over 220 songs on there. The Music folder on the computer when it is USB connected only shows 66. But when I stop the usb connection and look at the music player. All the songs are actually there. I don't get it. My phone is an Alcatel Onetouch (Virgin Mobile).
This is bad because if I get a new phone, my music won't be able to be transferred over.
When I check Android~Music it is empty.