I'm surprised you didn't get an "unrecognized action" error with the space.
Adding "W" just avoids DllCall searching for "PathRemoveExtension" before failing and reverting to "PathRemoveExtensionW" or "PathRemoveExtensionA". Aside from that, I think there is no benefit in this case.
Some functions have three versions: one with "W", one with "A", and one without either. In that case, omitting the suffix will not work as you might expect.
Newer Windows versions allow enabling long path support, which extends the soft limit of MAX_PATH for some functions. This only affects the "W" versions of functions, not the "A" versions, so in that case using the "W" version explicitly might circumvent the MAX_PATH limitation. However, PathRemoveExtensionW is limited to MAX_PATH anyway.
Again, the documentation says to pass a string of length MAX_PATH.
fileMp3 := "file.mp3" is not that long, but given what the function does, it is
probably fine. Maybe not. Some functions do silly things like zero out the rest of the buffer. Why risk it?
Note This function is deprecated. We recommend the use of the PathCchRemoveExtension in its place.