Here we go.....
You will not be converting MIDI to audio but... you will be using the MIDI to trigger your sound generator(guitars, synths, Drums) and recording the sound that it makes. I'm not fimiliar w/ the Powertracks deal... but there are programs that sequence and play both MIDI and wav files (aka Cubase VST, Pro-Tools, Logic, Cakewalk, etc...)
There is really a lot involved.... I have just now gotten to the " using MIDI to trigger recorded audio."
Get a good sound card... it will allow you to create your own audio.... and remember MIDI is not sound, but numbers used to trigger an instrument.