Try running this in a Terminal and posting the output here:
/usr/lib/jriver/Media\ Center\ 21/alsacap
with several usb connect-reconnect and then sound driver visible:
ubuntu 16.04 lts usb sound device trubbel hunting
1. in terminal$ aplay -l
**** List of PLAYBACK Hardware Devices ****
card 0: HDMI [HDA Intel HDMI], device 3: HDMI 0 [HDMI 0]
Subdevices: 1/1
Subdevice #0: subdevice #0
card 0: HDMI [HDA Intel HDMI], device 7: HDMI 1 [HDMI 1]
Subdevices: 1/1
Subdevice #0: subdevice #0
card 0: HDMI [HDA Intel HDMI], device 8: HDMI 2 [HDMI 2]
Subdevices: 1/1
Subdevice #0: subdevice #0
card 1: PCH [HDA Intel PCH], device 0: ALC283 Analog [ALC283 Analog]
Subdevices: 1/1
Subdevice #0: subdevice #0
card 2: M20 [MC-3+ Smart Clock USB 2.0], device 0: USB Audio [USB Audio]
Subdevices: 1/1
Subdevice #0: subdevice #0
2. in terminal:~$ /usr/lib/jriver/Media\ Center\ 21/alsacap
*** Scanning for playback devices ***
Card 0, ID `HDMI', name `HDA Intel HDMI'
Device 3, ID `HDMI 0', name `HDMI 0', 1 subdevices (1 available)
2..6 channels, sampling rates 32000, 44100, 48000 Hz
Sample formats: S16_LE, S32_LE
Subdevice 0, name `subdevice #0'
Device 7, ID `HDMI 1', name `HDMI 1', 1 subdevices (1 available)
2..8 channels, sampling rates 32000, 44100, 48000, 88200, 96000, 176400, 192000 Hz
Sample formats: S16_LE, S32_LE, IEC958_SUBFRAME_LE
Subdevice 0, name `subdevice #0'
Device 8, ID `HDMI 2', name `HDMI 2', 1 subdevices (1 available)
2..8 channels, sampling rates 32000, 44100, 48000, 88200, 96000, 176400, 192000 Hz
Sample formats: S16_LE, S32_LE, IEC958_SUBFRAME_LE
Subdevice 0, name `subdevice #0'
Card 1, ID `PCH', name `HDA Intel PCH'
Device 0, ID `ALC283 Analog', name `ALC283 Analog', 1 subdevices (1 available)
2 channels, sampling rates 44100, 48000, 96000, 192000 Hz
Sample formats: S16_LE, S32_LE
Subdevice 0, name `subdevice #0'
Card 2, ID `M20', name `
MC-3+ Smart Clock USB 2.0 '
Device 0, ID `USB Audio', name `USB Audio', 1 subdevices (1 available)
2 channels, sampling rates 44100, 48000, 88200, 96000, 176400, 192000, 352800 Hz
Sample formats: S16_LE, S32_LE, SPECIAL
Subdevice 0, name `subdevice #0'
this is frustrating. I will now maybe reinstall 14.04.4 an try again ....
I know usb unit supplier is not supporting linux, but mac is on the list and it worked perfect with 14.04.
thanks for the hints sofar!