Here is a live Celtic Frost recording from 1986 and an Onslaught rehearsal from 1984 (a double whammy!).
As expected, the Celtic Frost recording has a rough sound to it but you can still hear what's going on and it's just nice to hear a live recording back from when they were a great band. The Onslaught rehearsal also has a rough sound to it, but that is expected considering it was probably recorded on a boombox. For those who don't know or just are not familiar with early Onslaught, Onslaught started out playing a crust/punk proto-thrash sound, not too far off from Sodom's early demos, Hellhammer, and Impostor (Nor).
So here they are - nice, rare and raw for ya. Enjoy.
Celtic Frost - Live 7/10/86
Onslaught - 1984 Rehearsal