I need to find switch S4 for a Yeasu FT901 DM. WD92231 64000006 Where can I get this part?

I did a lot of digging around. Your FT-301 is a nice radio. However the service manual does not have the mic "pin out. " The Ham Forums do not have the mic wiring either. I did find that almost all mics are not wired the same as the Yaesu. Could you just buy a Yaesu mic? Sorry I cannot find this.

The best info I can find iis
radio mic
pin5 blue mic ground
pin6 red ptt
pin7 screen
pin 8 white mic

Call a technician to check it for you please

Try using a head set to see if the speaker is broken or disengaged, also check TX switch is not on.

try and purchased blunt S4 on ebay being sold for parts

Check on www.mods.dk. Pretty sure you'll find one there - you may have to join and pay a small fee but, worth it.

try www.yeasu.com/support

Click this link to download the manual of the GCS92.

http://iogear.com/support/dm/manual/GCS92#display

You can replace your audio and receive transistors with a new one or a compatible one and be back on the road again. the transistors are available from all ham dealers and are normally not too expensive.
do not try to fix something that ain't broke. if it is working, leave it.