Check your wiring, if you are not suffering from excessive wire losses both pairs should be about the same volume.

You can get a speaker selector / volume balancer but it is best to solve the underlying problem then use that for fine tuning the levels.

Thank you. The original wiring was in a soffit that gut removed during remodel. I spliced the wiring and set all spliced in junction boxes for future access. Wire quality looked good. I was thinking I would try reversing the polarity in case I messed up. 
Do you know what voltage I should see at the speaker? How much resistance is typical in speaker wire? 

Voltage varies with the volume of the sound being played. A test tone at full volume (55 watts into 4 Ohms) you’d see about 15 volts, with music it would be far less.

Wire resistance mainly depends on the size and length. This should got you the answer:

https://www.cirris.com/learning-center/calculators/133-wire-resistance-calculator-tablehttps://www.cirris.com/learning-center/calculators/133-wire-resistance-calculator-table

You may want to check the resistance of your speaker connection, with two on each channel it should be about 4 Ohms if the speakers are rated at 8 Ohms.

Thank you. 

Followup - speaker with no volume 6.6 ohms 

voltage at the amp and at the speaker end -seems to vary with volume. About 0.5 volts

attached the speaker directly to the amp - works fine.

suggests of what’s next? 
I have four Sonos ceiling speakers installed in the wall. 
two are fine two are not. 
wiring is in the walls so impossible to replace easily. 
thoughts? 
Thanks in advance 

jon

Make sure that the “phase” is correct on all speakers. This means the (+) and (-) connections match on all speakers. It’s easy to simply reverse a connection as a test. If this was your issue, there will be an instant, stunning improvement.

It’s more important that the connections match, than they be correct in an absolute sense. If you want to follow industry standards, connect a flashlight cell to a speaker and observe which way the woofer cone jumps at the instant of connection. When the (+) terminal of the battery connects to the (+) terminal of the speaker, the woofer will jump out.

Another reality check is that the resistance measurement from the amplifier to each speaker should be similar.

Thank you 

the phase on all four speakers is correct.

I did switch it just as a test. 

and correctly installed on the connect amp. 

I pulled one of the ‘dead’ speakers and with a short section of speaker wire - directly attached it to the amp. 
works fine. 
the ‘dead’ line shows approx 0.5volts with music playing. 
I get the same voltage reading at the amp. 
 

the measured voltage in the speaker line suggests to me it’s working and there is no break in the wire. 
 

any other thoughts? 
thanks

jon G

If the speaker works OK when wired directly to AMP and does not work when wired in place, there is obviously something wrong in the wiring.

In my phase test above had you reversed the wiring in only one speaker?

Yes I only reserved the speaker that was nonfunctional on its wiring. Agree something is wrong with the wiring. 
I was confused why I was still seeing the save voltage on the failed wiring as I see at the amp. 
I have a system of electrical gutter ( square conduit) that will carry A/C conductors. Can I successfully run a new speaker wire in that gutter with A/C wiring ? Shielded speaker cable ? 
Or is it a non-starter?

thanks 

Jon