German to English Medical Medical: Instruments

From a German medical report (Duplexsonographie der Beinvenen):

VFC (Vena femoralis communis) frei und komprimierbar, unauffällige A- und S-sounds.

I find it irritating that the English expressions "A-sounds" and "S-sounds" are used in the German sentence, but that I cannot find a suitable explanation or translation.

From all my research so far, "S-sounds" might stand for "spontaneous signals" and "A-sounds" might be "provoked signals", but I would be happy if someone could confirm, explain or correct this.