After transcoding for the first time, I hear the the chirp because I know it's there; after about three times, it gets annoying; after about five, the sample starts to sound like a squeaky bedspring!
As squeaky as they wanna be: (8 kHz mu-law audio):
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 allNot all lossy compression algorithms have to behave in this fashion; it is imaginable that, after being transcoded once, the sound doesn't degrade any further. Sadly, this isn't the case with GSM.
This not only means that you must be careful not to delete your GSM files while unpacking them; it also has implications for the use of GSM compression when audio streams have to me mixed together or edited during transport across a network. The more often a stream is decoded and encoded again, the worse the audio gets.