bahrd
No it's not at all unusual. Many factors, including the type of recorder, the recording medium itself, the microphones, how those mics are worn or used, volume settings, as well as levels of background noise (engines, air con, fans, etc) -- all contribute to dimished clarity of voices.
Other noises on the recorders are also very useful to investigators (such as audible cockpit warnings, alarms, horns, etc), so enhancing and otherwise cleaning up the sounds on the tapes is often done, and sometimes quite necessary.