Definitely the audio signal is too low.. Something's holding back the audio signal whether be loose connection or the audio amplifiers aren't amplifying the signal..
For loose connections, check all the path of audio signal for it's continuity.. And that includes the input jack, pots, etc..
Also for not amplifying properly, check all solidstates/tubes..
Or if you're still clueless of what to do.. let the expert do their thing..