Hello,
I am using Audacity 3.1 on a Mac and cannot get the input level slider to activate. I have included a short screencast of the problem. Does anyone know why this is happening and how to fix it? Thank you
For nearly all USB audio devices, there is no way for Audacity to adjust the recording level. Use the controls on the device instead.
Thank you Steve. I appreciate the info and quick reply
I just wanted to leave a fix I found.
I had the same issue with my Audacity 3.2.1 Windows application.
I was using a USB mic as well (Blue Yeti).
Fix: Audio Setup > Host > Change to Windows WASAPI
Hi Steve,
Thanks again for all the information you provide on this forum, this is most helpful !
My microphone is a t.bone SC450 USB, which I am using with MacOS High Sierra 10.13.6 for more than two years, and the recording level slider in Audacity was actually doing its job until the last version of Audacity (3.2.1) as far as I am concerned. I just checked it with 3.1.3 which was still installed on my Mac, I definitely can confirm it.
I’ve finally find the recording level in the 3.2.1 version thanks to this forum post : https://forum.audacityteam.org/t/sliders-for-recording-level-and-playback-level/65793/1 ('hope it will be back in the main bar soon, that is most inconvenient), but it is clearly not able to control the recording level anymore. Is there any hope to make it work for USB microphones again, or use some kind of add-on to make it work ? Otherwise I believe I’ll keep working with the 3.1.3 version, and will not been able to update anymore. 
Gaëlle
You could always use Audacity’s “Amplify” or “Normalize” effects after recording. If the volume control does work with an external USB device, it’s only doing the same as “Amplify” except that it is doing it on the fly rather than as a separate step after recording.
Thanks a lof for your swift answer, Steve.
I do use Normalize after all but I had the (wrong ?) idea that it would somehow improve the signal to noise ratio of my recording to adjust the volume level before recording, and avoid saturation.
Adjusting the level of the signal before it is converted from analogue to digital can indeed improve the signal to noise ration and avoid saturation, but for USB audio devices, the compute comes after the analogue to digital conversion. Some USB microphones have a “gain” (volume) control knob on the actual microphone for adjust the level prior to A/D conversion. Unfortunately, many of them don’t.