Lets say you always go with a risc.
It is always ilegal to use copyrighted work.
But some like Hasbro do not mind fan made projects involving MyLittlePony...
Depending on how successful and visible your project gets on the market, the more these companies will notice you.
You can always find soundtracks that can be used for free and sometimes require that you just add credits of the original author.
You can also check freesound.org wich has a bunch of free sound material even for commercial use.
If you feel too unpleased to use others works, you could also try buy a electronic keyboard piano and with a free public software called LMMS you can try make own soundtracks.
Theres a bunch of downloadable vst plugins that help you play a bunch of natural sounding instruments with it.