How do I get music onto my Micro SDHC Card?

[AC Question]

I currently purchased a 32GB Micro SD Card TF Flash Memory MicroSD MicroSDHC Class 10 memory card. I want to use it in my phone (LG Optimus L90) for music purposes and free up some internal memory space. I plugged my phone into the laptop (VIA USB) and tried to transfer music from my laptop to my SD card. When I would drag the file over to the SD card folder it says "This file may not be playable on your device", but it doesn't tell me why. It still copies to my card but it wont play on phone. Im having a very hard time with this.