I have a Bachmann On30 2-8-0 with dcc but I want to add sound do I have to uninstall the existing DCC to install DCC with sound.
short answer; yes.
Ton
You can obtain sound only modules, but you'd be much better off changing out the original with a full service sound module.
I've done what your asking about and will never do it again. In the end, I wound up replacing everything and going with a DCC sound module which would have been easier to begin with.
Depends on the specific set-up in the 2-8-0. Mine was DCC equipped, with the DCC decoder plugging into an NMRA standard 8-pin socket on the main board. I simply removed the stock DCC decoder and plugged in a TCS WowSound into that same socket. TCS makes an adapter cable for this. Simplest DCC installation I've ever done. Up and running with the new board inside of 5 minutes with no soldering required.
My 2-4-4-2 which I just finished had a different arrangement. It had the decoder built into the main board, with a 21-pin interface for adding the sound module to the locomotive. For the price of the sound module, I could get a full TCS decoder, so I pulled the factory electronics and installed the TCS decoder in its place. The connections on the stock board were labeled, so it was very easy to re-connect things to the TCS. That took about 30 minutes total.
Later,
K