Forget the vga box. Use one of your spare pc speaker sets.
The adapter your looking for is called: 2 RCA to FEMALE mini jack(3,5mm).
This is the cable you need:
http://www.audiogear.com/cgi-bin/shopper.cgi?key=AdptMniJ-LRRcaM&preadd=actionIt should be available in most in a local store. It's a pretty ordinary cable.
Connect the rca connector end to the amiga and one of your pc speaker sets to the female minijack.