ok.... we are close just follow the advice already given BUT in logical order, if you aren't comfortable with the multimeter then...

if you are lost just remember it is either mechanical (a speaker) or electrical (the amp or the signal to the amp) do this with only one speaker installed in the common chamber box

select one channel from the amp and drive a single speaker does it sound good?
select the other channel from the amp and drive the same speaker, does it sound good?

if both sound bad then either both amp channels are bad or the speaker is bad.

repeat this process with the other speaker

at this point you have identified either a bad speaker, one or two bad channels from the amp (both speakers could be bad but that is a little far fetched).

if it looks like it is one or both channels of the amp it could still be the signal to the amp...