13->14 microphone routing/mixing issues

Hi,

I upgraded from 13-stable to 14-stable last week, which seems to have
changed some aspects of the audio system.

My microphone is now being played out the speaker, resulting in
feedback if the microphone gain is above zero. I read the mixer(8)
man page but I don't understand what is wrong.

Although setting the microphone gain to zero quells the feedback,
doing so also means there is no microphone signal toward, e.g.,
teleconference software in the web browser.

How can I make the microphone usable again without feeding into the
speaker?

Speakers and microphone are attached to the "rear analog" ports.

% mixer -a
pcm0:mixer: <Realtek ALC888 (Rear Analog 5.1/2.0)> on hdaa0 (play/rec) (default)
    vol       = 0.86:0.86     pbk
    pcm       = 1.00:1.00     pbk
    speaker   = 0.74:0.74     rec
    line      = 0.01:0.01     rec
    mic       = 0.00:0.00     rec src
    mix       = 0.74:0.74     rec
    rec       = 0.35:0.35     pbk
    igain     = 0.50:0.50     pbk
    ogain     = 1.00:1.00     pbk
pcm1:mixer: <Realtek ALC888 (Front Analog)> on hdaa0 (play/rec)
    vol       = 0.84:0.84     pbk
    pcm       = 1.00:1.00     pbk
    speaker   = 0.00:0.00     rec
    rec       = 0.35:0.35     pbk
    igain     = 0.00:0.00     pbk
    ogain     = 1.00:1.00     pbk
    monitor   = 0.67:0.67     rec src
pcm2:mixer: <Realtek ALC888 (Rear Digital)> on hdaa0 (play)
    vol       = 1.00:1.00     pbk
    pcm       = 1.00:1.00     pbk
pcm3:mixer: <Intel Kaby Lake (HDMI/DP 8ch)> on hdaa1 (play)
    vol       = 1.00:1.00     pbk
    pcm       = 1.00:1.00     pbk

Thanks for your insights.