For posterity, I'll answer my own post.
After visiting the linphone irc channel, I learned from wabbits that the full command is needed.
address@hidden:~$ linphonec
Ready
Warning: video is disabled in linphonec, use -V or -C or -D to enable.
linphonec> help
Commands are:
--------------------------- help Print commands help.
answer Answer a call
autoanswer Show/set auto-answer mode
call Call a SIP uri or number
calls Show all the current calls with their id and status.
call-logs Calls history
camera Send camera output for current call.
chat Chat with a SIP uri
conference Create and manage an audio conference.
duration Print duration in seconds of the last call.
firewall Set firewall policy
friend Manage friends
ipv6 Use IPV6
mute Mute microphone and suspend voice transmission.
nat Set nat address
pause pause a call
play play a wav file
playbackga Adjust playback gain.
proxy Manage proxies
record record to a wav file
resume resume a call
soundcard Manage soundcards
stun Set stun server address
terminate Terminate a call
transfer Transfer a call to a specified destination.
unmute Unmute microphone and resume voice transmission.
webcam Manage webcams
quit Exit linphonec
---------------------------
Type 'help <command>' for more details or
'help advanced' to list additional commands.
linphonec> playbackgain 0
linphonec> playbackgain 1
linphonec>
Thanks!
Davis