There's got to be a way to do what you asking with recorded songs. I can do this on my PSR-550, and I know you can even do this with the PSR-740.. What you're wanting to do should be under the "mixer" settings. I know on the 740 there was a dedicated mixer button.. On my 550 the mixer button is next to the "voice change" button.. Check in your manual under using the mixer..

Squeak
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GEAR: Yamaha MOXF-6, Casio MZX-500, Roland Juno-Di, M-Audio Venom, Roland RS-70, Yamaha PSR S700, M-Audio Axiom Pro-61 (Midi Controller). SOFTWARE: Mixcraft-7, PowerTracks Pro Audio 2013, Beat Thang Virtual, Dimension Le.