I have the Flight Replicas Spitfire Mk V. My understanding is that the engine sounds for this aircraft are the ones from the Asobo default P-51 Mustang.
There is a sound improvement mod available for the Mustang. If I install this sound mod, will it automatically change the sounds that are aliased to the Spitfire?
No, it doesn’t work that way. Sound packs don’t overwrite base files, they simply tell the sim to load those instead of the base files. The Spitfire will still load the base files.
You can probably alias the sound pack to the Spitfire yourself though. I’m not sure how that works, but someone around here knows, I’m sure.