In Preferences/Audio, check that you have selected the ASIO driver for your audio interface (this is required). Also check that all audio inputs and outputs channels you want to use are active in "Active Input/Output Channels".
If you don't have an ASIO driver for your audio card, or if you want to use a mix of available audio in- and outputs (like a USB mic or a USB guitar cord plus the outputs of your audio card), you can try the ASIO4ALL driver. This is a free, general driver that can aggregate the WDM drivers for your different audio cards.
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