Well with what you have, you don't really need anything more, however a small mixer would help improve the sound rather than going through the player mic inputs. You don't need a mixer 'amp' as you already have an amp. The speakers I fear are designed for dj playback which the music portion will sound great, but when you pump live vocals through them, it could change their tune.
A small mixer like the Alesis Multimix 8 USB
http://www.musiciansfriend.com/product/ ... sku=630166
Would probably suit your needs just fine, PLUS it comes bundled with a decent recording software so you can hook your mixer up to the computer to record your performances.
You didn't state if you needed mics or not.