its not the recommended thing to do, but if it doesnt cause any problems then no worries.. if there is really no other option for wiring, give it a shot, if not, time to take out the seats and lift up the carpet.
If you run your audio next to any power source, even if it is only like 10awg, you have a chance to pick up noise. It might be a whining sound out of the speakers that gets louder as the revs go up, or just a constant humm, or a clicking sound.
Basically it is up to you, but many will claim it to "not be done correctly". A ground loop isolator can reduce these annoyances, but just like in Big Daddy where he covers all the spills up with newspapers, the mess is still there underneath.
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in this same vein... has anyone tried using copper mesh to shield wire runs? I happened across some jewelry related sites where copper mesh was made into tubes and used for decorative purposes... but given the right mesh density could you shield your power wire to contain EMI emissions?
Speaker wires are higher voltage than signal wires and nowhere near as prone to picking up inducted noise. You could always make the wires long enough to go either way and test it if you're really worried about it.