That makes me think I remember this happened to me a few months ago while at a music event !
If you can hear the little pop sound then the synth is ok.
I just tried intentionally interrupting the firmware upgrade just to check again. It seemed the solution suggested in the FAQ article did not work first, here is how I got it to work again :
-Turn off MatrixBrute
-Plug MatrixBrute USB on the computer
-Launch MIDI Control Center, select MatrixBrute as the device
-Turn on MatrixBrute while doing fast On/Off with the audio gate switch for a while
-After 5 seconds or so MIDI Control Center asks "Device is in Bootloader mode, please upgrade firmware"
-Select Upgrade (Not Download Latest)
-Locate the firmware file found here :
https://www.arturia.com/products/matrixbrute/resources-Carefully follow screen instructions (the MCC will ask for turning off and On the MatrixBrute at some point)
-The upgrade should perform correctly
Will see with my colleagues about adding this to the FAQ article.
Either way let us know, I am pretty sure this can be sorted without returning it.
Victor