The following signatures couldn't be verified because the public key is not available:

GPG error: https://dl.bintray.com/marshallofsound/deb trusty Release:

Ubuntu 18.04

$ sudo apt-get update
W: GPG error: https://dl.bintray.com/marshallofsound/deb trusty Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 379CE192D401AB61
E: The repository 'https://dl.bintray.com/marshallofsound/deb trusty Release' is not signed.

From the "software and updates" application (System/Administration if like me using the Traditional theme) select the "Other Software" tab and UNTICK the entry for marshallofsound as seen below (not the highlighted one, that's mega)

Uncheck

After that, exit and update as prompted. Either then through the command line or graphically from the software updater, update.