Slackware 12.1 on Acer Aspire 5710 laptop.
Intel HDA audio.

There used to be a mixer control for microphone (or 'Mic'). A few months ago it vanished. The microphone works, but is at a fixed level and always with annoying playback through the speakers. I cannot change the recording level as there is no mixer control.

$ amixer | grep mixer
Simple mixer control 'Master',0
Simple mixer control 'PCM',0
Simple mixer control 'Line In Boost',0
Simple mixer control 'Mic Boost',0
Simple mixer control 'Capture',0
Simple mixer control 'Beep',0
Simple mixer control 'Digital',0
Simple mixer control 'Input Source',0
Simple mixer control 'Internal Mic Boost',0

$ amixer set Mic 50%
amixer: Unable to find simple control 'Mic',0

There is a 'Mic Boost', but no 'Mic'

I will swear by all the daemons of Linux that I used to have such a control! How can it just disappear, and how can I get it back?

Does this look right?

$ lsmod | grep snd
snd_seq_dummy 6788 0
snd_seq_oss 33408 0
snd_seq_midi_event 10240 1 snd_seq_oss
snd_seq 50928 5 snd_seq_dummy,snd_seq_oss,snd_seq_midi_event
snd_seq_device 10380 3 snd_seq_dummy,snd_seq_oss,snd_seq
snd_pcm_oss 40480 0
snd_mixer_oss 17920 1 snd_pcm_oss
snd_hda_intel 367756 10
snd_pcm 70788 4 snd_pcm_oss,snd_hda_intel
snd_timer 23176 4 snd_seq,snd_pcm
snd_page_alloc 11400 2 snd_hda_intel,snd_pcm
snd_hwdep 10756 1 snd_hda_intel
snd 51748 25 snd_seq_oss,snd_seq,snd_seq_device,snd_pcm_oss,snd_mixer_oss,snd_hda_intel,snd_pcm,snd_timer,snd_hwd ep
soundcore 10080 1 snd

Strange. Did you do any kernel upgrades etc? And do you dual boot into another OS where you can verify that this is not a hardware problem?

Strange -- it is!

No upgrades to kernel. I rely on this machine for work, so once set up I leave it alone and don't change anything. Hence the old version of Slackware --but hey, if it ain't broke, don't fix it! ...except that it somehow broke itself in this regard.

No, I don't dual boot. The laptop came with Vista, which barely ran. I just deleted the partition table. It runs great on Slackware - but what doesn't?

Hardware? You think the mixer could be broken, but mic input still works - sort of? Hard to see how just that one control could go missing, but the rest of the mixer is working normally.

Ok can you at least use a live distro to check that the hardware is OK? If it is, I can only think of a corruption on the disk, causing some software to fail. If so, you may need to reinstall certain packages.

That would be a good idea to try if I had one (live CD) with me. Unfortunately I don't. I should explain; I am on a ship at sea and have only the slowest of internet connections -- varying from a few kB/s at best to less than one at worst. Large downloads are impossible.

It would take a long time, but I could perhaps download a few packages overnight when there is less usage so the speed is better. But, which packages might be responsible? I already replaced all of the ALSA kernel modules with new ones (latest version) compiled from source code. It changed nothing.