How can I determine run time of a 12V vehicle amplifier using an 18V 5Ah battery
Ah / A = Time in hours, so choose a power amp, find out its current draw in amps and plug in the numbers...

You can also do Ah / hours = amps which gives 5 (Ah) / 8 (hours) = 0.625 amps to run your power amplifier, a small one then.

Note I ignored the voltage difference and also that you probably won't get the full stated capacity of the battery in Ah..