The temporary solution is to avoid having any empty channels – at least while getting your mix right. So instead of having (for example) tracks 1-5 filled with audio files and then say, tracks 14,15 and 16 filled with audio files. You want to avoid having the gap between 5 -14. So in order to get your mix the way you want it you should move the files from 14,15 & 16 to tracks 6, 7 & 8 (eliminating the gaps). Things should then work as expected. Get your mix levels the way you want them, mark down the settings and then move the relevant files back to where you need them and make the adjustments as per what you marked down. Now it wont sound right in utool but it should perform correctly in the playback device.