Conversation so far:
how did Lee de Forest amplify the audio frequency?
by feeding it into a second tube for additional amplification.
did he have any problems?
It was found that de Forest's "gassy" version of the Audion could not handle even the relatively low voltages used by telephone lines.
how did he fix it?
by improving the tube's design, it could be more fully evacuated, and the high vacuum allowed it to successfully operate at telephone line voltages.
Are there any other interesting aspects about this article?
AT&T, through a third party who disguised his link to the telephone company, purchased the wire rights to seven Audion patents.
how much did they purchase it for?
for $50,000.
what did he do after?
In 1915, AT&T used the innovation to conduct the first transcontinental telephone calls.
Did they use it for anything else?
CANNOTANSWER.

Question: what made him amplify the audio?
Self-contained rewrite: what made Lee de Forest amplify the audio?