Add an option or mention how to test all the speakers on the system.

Currently, working back from what the man page says, one needs

for i in $(aplay -L|perl -nwle '/(.*):CARD=([^,]+)/ && print "$1:$2";'|sort -u)
do speaker-test --device $i --nloops 1; done

And that creates a lot of errors, as not all the aplay -L devices can be
used.

Anyway, the idea is: try making a noise that can be heard somewhere.
E.g., it is night time.
My house has 25 windows.
I will make a noise inside it,
and stand outside, wondering what window the noise came out of...

So running the above command, when the user finally hears a noise,
he can look quickly at the screen to see which --device made it.