The constant throb of dirty static bass.
The slight raise in pitch when entering the safety of the neutral zone.
The frantic nibbling of the wall around the Quotile.
The panic inducing siren of an incoming Swirl.
The satifying sound of a successful hit with your Zorlon Cannon.
Not many games from 1981 hold up well today, especially in the audio departmart. But Yars' Revenge for the Atari 2600 was brilliant back when I was a kid and it still is today. Grab an emulator and a rom and see for yourself. Try the self titled debut LP from Zombi or the 2003 Demo LP from Steve Moore for musical accompaniment.