USB Sound Assertion Problem

I have an upgraded Mac Pro 5,1 (by upgraded I mean dual 3.46 GHz 6-core Xeons). The system boots from 2 striped SSD's housed in a Sonnet Tempo SSD pro Plus PCIe 2.0 Card and is presently running macOS Sierra 10.12.3. As you might imagine the system boots extremely quickly and application response to good. Data storage is via 4 RAID 10 2TB disks in the 4 normal bays. More recently I've installed Sierra on the RAID setup which gives me an alternative way of booting the system in the event of SSD failure - see below.

There are a number of issues that I do not fully understand and where I'd welcome advice:

(1) According to Sonnet's technical support, "Option Boot" is not supported which means accessing a different boot volume / partition (e.g. for disk checking) is anything but straightforward! I'm told I need to install rEFit or use the StartUp disk in User Preferences to switch to different boot volumes. Using the StartUp settings in User Preferences works fine UNLESS there's a problem booting into the striped tempo card in the first place and I haven't managed to work out how use the rEFit suggestion yet!

(2) This brings me to the nub of the problem. Every month or so the system won't shut down properly unless pressing and holding the start button. I've learnt the hard way that when this happens the best thing to do is to immediately re-start the system with Cmd S (single user mode) and run Fsck - fy (twice). When I do this I get a long list of USB Sound Assertion errors on the first run:

** Checking catalog file.

USB Sound assertion in /BuildRoot/Library/Caches/com.apple.xbs/Sources/AppleUSBAudio/AppleUSBAudio-307 .7/AppleUSBAudioControlInterface.cpp at line 357

... Repeated for lines 997 to 1403

Invalid node structure

(4, 33362)

Invalid sibling string

(4, 33362)

** Rebuilding catalog B-tree.

The second time I run Fsck the report, the changes made on the first run seem to fix the problem and the system boots ok.

Any idea what might be causing the problem in the first place? Do I need to worry?

Many thanks for any help.