Good afternoon, I hope I can help you
First thing to do is remove the set and check the wiring plug connection at the rear is ok.
You need to make sure you have your radio code first though as it will lock out if the power connection is removed when checking.
If the connections look ok then the next step would be to try borrow one to see if that worked properly.
If it did then your one needs repaired or replaced. If the replacement had the same fault then you need to check out the wiring to each speaker for damage or bad connection.
One speaker being blown would not reduce the volume to the others unless the balance and fade controls had been set to the blown speaker. I have seen this a few times with customers who didn't know how to set the controls properly.
A blown speaker could have a tear in it but it could also be a fault with the wiring on it or a blown resistor if fitted.
A visual inspection may not be good enough. You need to check resistance of each wire from the rear of the set to each speaker.