Mikf: correct. Additionally, it is *only* 2 channel audio, so you can't even play back audio (like and Audio style or .wav multipad) while recording audio.
Ajag: You can load the midi into the PSRs970, and you can save the audio to a usb stick, but good luck getting them to sync (there's no built in functionality for that) *unless* you convert your wav files into an audio style, *then* you can sync
Keep in mind you also need to merge all channels of audio into a simple 2 channel (left right) audio file; the s970 being only 2 audio channels, it can't take simultaneous wav files, or play back while recording audio, or do overdubs. With Tyros/Genos/CSP, which are 4 channel audio, you can play back and record simultaneously (which allows for infinite ping pong recording)