In order to improve the transmitted audio from a IC-756 most off these radios have a tiny switch next to the 455kc wideband x-tal filter.
Open de bottom side off the radio and turn it so you can read the text on de 455kc standard wide ssb filter. Next to te right down side off the filter you will find a tiny smd switch with W-N printed on de pcb. Change the switch position to wide.
Make sure you use a good mike! A bad one will sound even wourse!
And be carefull not overload the ALC. Otherwise you might splatter!!!
BTW Early numbers (below 01900) do not have this secret option!