On 7/27/20 8:13 AM, Greg Woods wrote:
> mythtv@seveneves ~]$ amixer -c0 controls
> numid=19,iface=CARD,name='IEC958 In - Output Jack'

That's promising.  Try:
amixer -c0 sset 'IEC958 In - Output Jack' on

[...]
numid=19,iface=CARD,name='IEC958 In - Output Jack'

[...]

[root@seveneves sbin]# amixer -c0 sset 'IEC958 In - Output Jack' on
amixer: Unable to find simple control 'IEC958 In - Output Jack',0

[root@seveneves sbin]# amixer -c1 sset 'IEC958 In - Output Jack' on
amixer: Unable to find simple control 'IEC958 In - Output Jack',0

--Greg