There are many solutions! How about cascading the machines?
Meaning, to route the audio output of every machine to the next one's audio input?
Just like:
Computer #1 Line-Out connected to computer #2 Line-In,
Computer #2 Line-Out connected to computer #3 Line-In,
[...],
Computer #(n-1) Line-Out connected to computer #n Line-In, and, finally,
Computer #n Line-Out connected to the speakers audio input.
Or, by the use of an external stand-alone audio mixer of a capable number of audio inputs.
Or, using an audio card with as many audio inputs needed, installed to the computer connected to the speaker set.
-George