There is just the one mute. It mutes both channels. On sheet 5 top left there is a handy mute signal truth table. The tip path mute has a pass FET Q5 and a shunt FET Q6 ( for extra attenuation). To pass the signal Q5 will be low resistance (gate around 0v) and Q6 high resistance (gate negative). For sure R67 should be the same signal as U3 pin so did you check both ends of R67?
That resolved check for signal on the drain of Q5 ( not muted). If the gate signals are OK then one or both of the FETS must be bad. You can use an ohmmeter between source and drain to check if a particular FET is high or low resistance.