Before the post: the output device was selected; so was Software Playthrough; and Linein was running.
Yes, I read what was in the manual.
The problem seems to be with the mixer toolbar. Its options are: disable meter, stop/start monitoring, vertical stereo & linear, but the sliding scale is not available.
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It is always helpful to state what you have tried already.
You are talking about Meter Toolbar when you are talking about those options.
The output slider is in the toolbar I posted an image of (Mixer Toolbar):
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Check in View > Toolbars > and make sure there is a check mark by Mixer Toolbar then you should be able to see it.
Check in System Preferences if you have muted the system sound.
Then just try recording. Are you recording blue waves like this:
If you are just recording a flat line then you cannot hear the recording because you are only recording silence.
In that case choose the USB Audio CODEC as input device in Audacity’s Device Toolbar. If USB Audio CODEC is not visible in the input device box, click Transport > Rescan Audio Devices then try recording again.
Note that it should not matter, but you stated that System Sound input was set to Audio Line-in. If that wasn’t a typo, try setting that to USB Audio CODEC too.
If the turntable is not recording from the USB cable, you will also not be able to play it through the computer speakers without recording.
Does the turntable have red and white leads for connecting to speakers? It is simple (with an adaptor) to connect those leads to the Mac audio input and record from that.
Do you have any relatives or friends who are familiar with computers and could give you a hand in person?