My VX-6R, and that of one of my friends, had very low audio on 70cm out of the box. I received some very bad signal reports on a local repeater while doing side-by-side comparisons with my VX-5R and VX-7R, both of which received good reports. Not only was audio low on the VX-6R, but I was unable to "hold up" the repeater.
The fix for this is to increase both the transmit deviation and PL tone deviation using the service menu as follows:
Set the VFO to 440.000 and turn the radio off.
While holding down BAND-Internet-0, power on the radio (Internet is the button on the lower left of the keypad, with a D next to it).
You will see "ref" at the top of the display, above 440.000. Rotate the channel wheel clockwise until you see "dEU" at the top of the display.
Press the V/M key once to enter the set mode. The transmit deviation setting will appear at the top of the display (mine was set to 62 at the factory). Write this setting down in case you need to back out the changes you are about to make.
Rotate the channel wheel to change the setting to 85. Press the V/M key to exit the set mode. dEU appears at the top of the display.
Rotate the channel wheel clockwise one step. You will see 100 at the top of the display. This is the PL tone deviation setting.
Press the V/M key to enter the set mode. Write down the factory default setting (mine was 62).
Change the setting to 75. Press the V/M key to exit the set mode.
Press the HM/RV key to save the changes. If you make a mistake and don't want to save the changes, just power off the unit instead.
I arrived at these settings after much trial and error. The "right way" to align the radio would be to put it on a service monitor and adjust the settings while measuring the deviation.
Be that as it may, by making small incremental adjustments to one setting at a time, and getting signal reports after each one, I was ultimately able to arrive at the settings which sounded the best to others. My VX-6R now gets signal reports that are equal or better than my VX-5R and VX-7R.