The fans at web site
transformertoys.co.uk recently acquired some cassette tapes that include some recording sessions for early G1 episodes. Here's the first one that they uploaded:
https://www.youtube.com/watch?v=vwOQhJ_UA5E
There are scenes and dialogue that were evidently cut from the broadcast version, including some "new" banter between Wheeljack and Bumblebee, more generic Decepticon jets, and a repair scene for Bumblebee following the attack en route to Iacon.
Some people seem to think this represents the original voice session but I tend to disagree. For one, Soundwave's voice already has vocoder effects applied, which would be done in post-production. Also, there is an echo effect applied to all the character dialogue. The voices as they appear in the final episode don't have a reverb effect, so I'm thinking this was an early attempt at creating robotic-sounding voices for the cast and was eventually rejected in favor of the more familiar layered flanging effect. (I wonder if the fact that Challenge of the GoBots did use a reverb effect for the Guardians had something to do with this change.)
Discoveries like this are always so fascinating, and I'm eagerly anticipating more of these updates.
Zob ("Autobot alert! Transformation required!")