Have a read on the bloke who developed his own Bill Paxton themed pin.
He used propeller chips which had some support for pulling sound files from an SD card.
He described his techniques and I think gave out the source code. Maybe this will help you?

http://benheck.com/bill-paxton-pinball-making-of

Depending on how much sophistication you need in your sounds I suppose you could also just grab an old sound card from an existing pin and just use whatever sounds are on it. They are just driven by putting a word value on the input lines to select a sound.

Good luck wit the project.