How to set default to 8-bit

need to reset the default from 16-bit to 8-bit at 22khz

No you do not need to set the default to 8-bit, and if you did then you would need to use a different app because Audacity tracks must be 16-bit or 32-bit floating point. The default is 32-bit floating point, and there is no good reason to ever change that unless your computer is dangerously short of disk space.

If you want to create an 8-bit file, create your audio as a normal 32-bit floating point track, and then export it as 8-bit.

22kHz is not a standard sample rate. You will probably want 22050, which you can set in the export options.

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Also note that simply changing the sample depth to 8 bits will not make your audio sound like the old-school “8 bit” sound, in case that’s what you’re trying to achieve. All it will do is add more audible noise to the audio (8 bits has about 48 dB SNR, which gives you quiet but audible noise, while 16 bits has about 96 SNR, which gives you inaudible noise).

If you are after the “8 bit” sound, try out software like GitHub - LemonATsu/pop-to-8bit: A tool for converting pop song into 8bit chiptune song.. I haven’t used it but it looks good.