no usb microphone audio in linux mint 19
 

any help appreciated

it could be something simple like your mic is muted and so volume inputs are not high enough to record anything
consider this image after running

https://imgur.com/Vs9vcfx

and pressing F6

alternatively if you know you have only one internal device you could try
The mic input shows Mute against by the MM keys

use the right arrow key to get to that control
press the m key to unmute

NOTE THAT...for some distros....you press the SPACEBAR to unmute so YMMV

then raise the volumes using either page up or arrow up
and then test your mic

let me know if you have any further troubles