System: MacPro 64GB RAM 3.5 GHz 6-Core Intel Xeon E5 running Yosemite 10.10.5 (14F2511)
Firefox version 60.0.2 (64-bit)
When using any other website including YouTube, google drive, Facebook, etc. sound is normal and plays in stereo through both the internal sound card and external Core Audio devices.
When using SoundCloud sound is in mono.
This does not happen in Safari or in Chrome.
Mike C. NYC