The Delta does not use the Windows mixer.
It might be that you are using the wrong setting for the input level. This is from the Delta 66 manual:
"The software switches in this section allow the user to match individual input levels and global (as in ‘all’ or ‘across the board’) output levels to the operating signal levels of the external audio equipment. Three level selections are available: +4dBu, ‘Consumer,’ and –10dBV. The ‘+4dBu’ setting is the least ‘sensitive’ of the three settings, and ‘–10dBV’ the most sensitive. Therefore, the ‘+4dBu’ setting has the most headroom and can accept the hottest signals of the three settings.
"Consult the user guide of your external audio equipment regarding your equipment’s line level. If for instance your audio equipment is consumer or semi-pro, and you find that its input level is a little too hot for the Delta 66’s ‘-10dBV’ setting, try switching to the ‘Consumer’ setting. Conversely, if your –10dBV gear is receiving a signal from the Delta 66 that is too hot, try switching the Delta 66’s output levels from ‘Consumer’ to the ‘–10dBV’ setting."
The Delta manual explains how to change these settings in the Delta Control Panel utility...
