I don't know of a way to do this, specifically within AMS, other than perhaps finding the image for "dB" and clicking relative to that... which I can't imagine you would want to do!
The input volume can be adjusted by Applescript using, e.g. set volume input volume 50 (or, embedded in a shell command, osascript -e "set volume input volume 50").
See this discussion on StackExchange.
The forum thread Set system volume via script/KM variable might also be of interest, although it is concerned with output levels.
If you can say more about which audio device you are using, the version of MacOS and, not least, the reason why you want to do this, someone may be able to give a better answer.