In wave interference, the amplitude can be increased by in-phase addition or reduced by out of phase addition, or some combination dependent on phase angle.
If the modulation is to large your bandwidth will be to wide in other words the frequency shift will be larger than normal that will result in a distorted audio signal at the receiver
The amplitude of a wave doesnt affect the amplitude because amplitude is affected by force energy. If more pressure was applied on the wave it would be stronger causing greater amplitude.