Microphone problem, Lenovo ideapad 720S-14IKB, Ubuntu 18.04

I have a Lenovo ideapad 720S-14IKB laptop with Ubuntu 18.04 and the built-in microphone does not seem to work.

In Windows 10 (also installed on the same laptop, dual boot) the microphone works fine.

The way I have tested this so far in Ubuntu is to use "Sound recorder" to do a recording, then the resulting recording is just silent no matter how loud I scream at it.

Since the microphone works in Windows I guess it should be possible to make it work also in Ubuntu -- what can I do to troubleshoot?

EDIT: Now, a few months later, the microphone has started working. I still don't know why it did not work earlier or what changed so that it started working. I did not knowingly change anything other than installing the normal updates using Software Updater. Anyway, the problem seems to have fixed itself.

EDIT 2: Now (8 Feb 2019) the microphone has stopped working again. It was working a week ago but now for some reason it is no longer working. As before, it works fine in Windows 10 but in Ubuntu it does not work. Any suggestions for how to troubleshoot would be much appreciated!