relay switch selection
This isn't really an answer for the question, but the comment field is too small. 

Speakers don't actually use DC currents. Instead they use AC currents, so it could be that your measurement is incorrect. Sometimes some multimeters will detect some AC voltages as DC, so it's best to be sure when measuring. 

So a good way to test this would be to generate a signal which would be easier for multimeter to read. A nice program is Two channels frequency generator  which can be obtained here.

Next check the output characteristics of your speaker. If it can make 50 Hz or 60 Hz sounds, open the generator, set left and right frequency to 50 Hz or 60 Hz and click start (or if you can't connect the system to a computer, record the sound and then play it on the speaker) and be sure to set the volume to maximum both on the device and in program and on computer output.

So why 50 Hz or 60 Hz? Well those frequencies are commonly used for mains power and most multimeters are made so that they can easy and correctly read voltages at those frequencies. A good quality multimeter should be able to read voltages at other frequencies too, but this way you'll be sure that you get the correct reading. 

If the speaker can't produce those frequencies, then try with whole number multiples of them.

Next, why frequency generator? Well, normal music produces complicated waveforms which change very quickly and are quite complicated. A single tone which is made by the frequency generator will be much easier for multimeter to read. 

In the end check available settings on your multimeter. Good meters will have AC+DC voltage measurement option which will give you the real voltage at the speaker. If yours doesn't have such setting, try measuring both AC and DC voltage. Somewhere I read that you should calculate the actual voltage using following formula: \$V_r=\sqrt{V_{DC}^2+V_{AC}^2}\$. That should give you the real effective voltage.