I had to return to this subject when I was planning to get Reason Essentials (I'm trying out it in demo mode) to replace Audacity as more advanced live recording program. For some reason Reason accepts only ASIO input and while trying to solve the high pitch noise issue of Multimix I made two observations:
1. When input for Multimix is turned off there is no noise
2. When output for Multimix is off (using internal/soundcard output) but input is on there is no noise
So you can either monitor with Multimix or record with it. You can't do both at the same time or there will be that high pitch noise. The problem with output is that you can listen it or you have to turn on software monitoring which has noticeable latency, essentially making it unusable for recording.
What I know about the noise is that it's generated by USB port actually and it's most likely a feature of some products out there. I would be very surprised if I got any money back from Multimix so it's really not even worth trying.
It seems to be obvious that some areas of technology suffers from these kind of problems. Money can fix everything as usual, but we all don't always have that luxury. However I for sure have learned a lesson and wont be fooled again. My next usb interface shall work as expected or it will be returned promptly.