The best sound decoder available right now that fits (more or less; sometimes you have to thin out the sides of the shell a tiny bit) in an N scale diesel is the ESU LokSound Select Micro. For the SD70ACe, you will need sound file #73825, which is the sound file for the 16-710G3C-T2 prime mover. Whomever you buy the decoder from should be able to install this file before sending it to you.
I've seen photos of an ESU install in a Kato SD70 somewhere; I just can't remember where. But the chassis is clearly long enough to mount the ESU and a 8x12mm or 9x16mm speaker.
John C.