How do I get Audacity to recognise Lexicon Alpha as the external interface?
Plug in the Lexicon Alpha first. Check in the Windows Sounds control panel that Windows can see the device and is receiving a signal when you put sound into the Lexicon Alpha. Then launch Audacity and select the Lexicon Alpha as the recording / playback device (see: https://manual.audacityteam.org/man/device_toolbar.html)