What kind of target audio level are you after?? The line output from the Amiga is approx 1V RMS which is a fairly standard input level for most TVs, amplifiers, etc.
Or is the output impedance of the Amiga a problem? It's 1k ohm, so if you connect a 1k ohm load to this, the AC voltage halves. Most TVs and amplifiers have an input impedance of 10k-47k ohms though, so that's unlikely to be a problem.
From a 9V battery, you could easily build a small amplifier capable of a voltage gain of 10dB (3 times, so for a Vin of 1V then Vout = 3V RMS).
If you want to have a go at building something yourself, I can scribble something up and shove it in the scanner for you.
