The Darktide soundtrack will be available to owners of the Imperial Edition, regardless of when the Imperial Edition was purchased.
We were experiencing some issues with it not being available to players initially, but you should now be able to locate the soundtrack by:
1. Restart the Steam client
2. Navigate to your Steam library
3. Select the 'All'/'Games' dropdown on the upper left-hand side and select 'Soundtracks'
4. The Darktide soundtrack should be listed in there and available to download to your chosen directory