I see that you set ADMUX to 0. That means that AREF is used as the
ADC's voltage reference. Do you have a reference voltage connected
to pin 62, AREF? If not, you need to make sure you apply at least
the maximum voltage that your ADC inputs will see (but not more than
the VCC of your board). It's usually safe to just apply VCC to AREF
(and make sure your inputs don't exceed VCC).