[ALUG] No input levels?

I've had this problem for a while and tried a few things, but before I finally give up I thought I'd ask ALUG about it. Without headphones plugged in, there is a clear input level showing on the sound settings. With any headphones plugged in, however, none. So I can watch videos or whatever, but it's impossible to use video conferencing tools as no-one can hear me. Ideas? Mint 19.1. on a Dell Latitude laptop. Bev.