Kurt-
Yes, you can add railsounds to an older locomotive, even without TMCC. The Electric RR Company offers several modular kits to retrofit older locomotives; I would recommend the RailSounds Commander kit, which would add steam or diesel Railsounds to any conventional locomotive. Since it is a modular upgrade and has a relatively small footprint, it can be installed in almost any locomotive, or even in a piece of rolling stock. Installation is very easy too - simply splice a few wires together and you'll be up and running (Electric RR's instructions are some of the best around).
Electric RR Company does sell the kits direct, but Boxcar Bill on the forums offers the kits at a discounted price.
Hope this helps!
-John