Conversation so far:
how did Lee de Forest amplify the audio frequency?
he came up with the idea of strengthening the audio frequency output from a grid Audion by feeding it into a second tube for additional amplification.
did Lee de Forest have any problems with amplifying audio frequency?
de Forest's gassy version of the Audion could not handle even the relatively low voltages used by telephone lines.

Question: how did he fix it?
Self-contained rewrite: how did Lee de Forest fix the problems with amplifying audio frequency?