I realize this thread is almost 2 years old, but in case the OP hasn't gotten hold of the mentioned voice card: replacement voice cards have been re-designed and are available at the DMX files website for US$ 98 each
As for the cowbell, having studied the documentation for modifying 1554 voice cards
, it can be configured to have two pitches (as opposed to just having two sound levels) if I recall correctly. I've heard the two-pitched DMX cowbell in many recording, so you might want this instead of just a single pitched one. An example is "Let it all blow" by The Dazz band
(listen from around 3:10)
I've got a DMX myself which I haven't used for years and there are many sounds I'd like to have, but getting hold of dedicated sound cards for each sound will add up in cost. Ultimately I'd have each card to be configurable for various sounds, but I'm not sure if that can be done simply with switches or if each card needs to have specific components for the particular sounds (I need to re-read that modification document linked above). It would be great if an EPROM expansion board (taking multiple voice EPROMs) could be used, and an internal switch could be used to choose between say a DMX bass drum, Linn Bass drum (boy do I regret selling my Linndrum back in the 90s!, but I digress), Electronic bass drum etc. and similarly for the snare, toms etc. instead of pulling out the EPROMs each time.
EDIT: look what I found: Someone selling the voice card you're looking for priced at US$ 99