Splashtop team discovered a reason why some machines running the Linux Streamer cannot capture audio.
This would happen if the audio capture device is muted.

Here's how to check whether the audio capture device is muted or not:

(1)  grep -i 'capture device' ~/.config/splashtop-streamer/SPLog.txt
The response to this command should display a name such as  "alsa_output.pci-0000_00_14.2.analog-surround-51.monitor"

(2)  pactl list sources
Find the matching name, and its "Mute" property.
If "Mute:yes" then the capture device is muted.

If muted, use this command to un-mute it.
(3)  pactl set-source-mute 'alsa_output.pci-0000_00_14.2.analog-surround-51.monitor' 0