Are you talking about PA announcements made by flight crew to passengers on commercial aircraft?
The flight deck is a noisy place, and the interphone (the handset they used to talk to cabin crew or make announcements) is not designed primarily for studio-quality sound. But it really depends; I often hear announcements that are surprisingly smooth and fine-sounding.
If you’re talking about air traffic control recordings that you might hear online, it’s because the communication link uses old-school VHF radio transmission with a narrow voice band and which is very susceptible to interference (much like old AM radio).
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