I used this to make the sounds:
GinSing - a synthesizer shield for the Arduino
It's pretty nice and has a 1/4 output jack. It depends on how much flexibility in sound you need - for a hobbyist like me it's great, I'm just happily using the built in sounds. If you really want full control over the sound it may not suffice at all. Or it may. I haven't plumbed it's depths.
GinSing - a synthesizer shield for the Arduino
It's pretty nice and has a 1/4 output jack. It depends on how much flexibility in sound you need - for a hobbyist like me it's great, I'm just happily using the built in sounds. If you really want full control over the sound it may not suffice at all. Or it may. I haven't plumbed it's depths.