I had good luck with Lady Ada's waveshield which can be ordered from her WEb site, http://www.ladyada.net/make/waveshield/index.html
Also, as Marco_s touched on, the easiest solution was to use two Arduino's - one, an Uno, that the waveshield sits on and is dedicated to it. And an older one I happened to have that only has to process my inputs (a bunch of reed switches that respond to a passing magnet) and turn them into clean 100 millisec output pulses. These are fed into the Uno inputs through 500 ohm resistors to start each of six songs. The two Arduinos use the same power supply and shared ground.
Some of my "songs" are actally voices, etc. I use "Audacity" to process the material and make it compatible with the waveshield SD card. To program IR sensor inputs should be very similar to what I have done and I believe sample code for this is available in "Arduino Examples".