I was just wondering if I can put songs (that I bought legally) into my guild If so, can you please tell me how?

Just because you bought it legally doesn't mean you can upload it onto the Internet.

Your best bet is to just find already-uploaded files, then use this code:
<IMG height="1" loop="infinite" dynsrc="PASTE URL HERE" width="1" autostart="true">

I don't know if that code works for guild layouts (I don't even know if Neopets allowed music in guilds now), but give it a try.

You could also use the bgsound tag, like so:

<bgsound SRC="mymusic.mp3" LOOP="infinite">

This will make your audio file the background music.