Sierra, with the flat acid battery - no more batteries.
I had an Accy K27 with sierra sound (I had a few Soundtraxx Sierra equiped locos) and when the little lead acid batteries died and became non-existent to buy, I used a 4x1.5 AAA NimH rechargeable battery 'stay alive' pack instead (it enables sound when just starting up motion and also ancillary sounds when motion stops). Everything worked fine.
I then started to replace all of the Soundtraxx cards with Phoenix boards.
The sound was even better than the Soundtraxx Sierra boards.
I bought pre-owned Phoenix
BigSoundTM 2K2 boards, and as the loco sounds could be up-loaded (using the Phoenix programmer), it did not matter which sounds the second-hand jobbies came with.
The 2K boards are also compatible with DC and DCC (the sounds can be controlled with a DCC controller).
When I began to incorporate DCC into my layout, I tried just adding a motor DCC board to work with the Phoenix 2K2 boards but eventually I wanted a simpler route and started to buy ESU V3.5 DCC motor with sound boards cheap. I also purchased the ESU programmer and was able to upload different loco sounds. The quality of the sounds was easily on par with the Phoenix boards.
As the ESU DCC boards will also happily work on analogue, there is no problem using them on analogue lines.
Phoenix have brought out the SM18 DCC which is motor and sound but they are not cheap.
I would highly recommend ESU DCC motor/sound boards.... especially if you can get Version 3.5 or V4.0 cheap.
I do have spare, a few Phoenix 2K2 boards and maybe an ESU V3.5 or two.