I agree that less is more with these things.
A couple of products you might want to look at are
Voices by G. Patrick Redford and
Polter by Phanthomas.
The latter in particular might suit you. One of the downsides for using it in a séance situation is that you cannot control the timing exactly. For you, this will not be an issue at all, and it only requires relatively simple props to perform.
This review is very negative, but I think that's probably because the idea is pretty much useless for most situations where you would want to achieve this effect. However, in your case, it might actually work well.
http://www.talkmagic.co.uk/ftopic3825-0-0-asc-.php