This is one of those great questions I’ve also wondered. There are GUI applications to deal with standby mode and such, but I don’t know what to tell you for command line. I have a number of low power and standby functions disabled by default (including through rc.local), but the monitor will still blank after a certain time unless I use the GUI app.
The command line tool “gsettings” may be what you want to change some GUI details, but I couldn’t tell you for sure. I use this after an install (this is nothing to do with audio, but I suspect a similar setting exists which the GUI controls work with for standby modes):
gsettings set com.canonical.Unity.Lenses disabled-scopes "['more_suggestions-amazon.scope', 'more_suggestions-u1ms.scope', 'more_suggestions-populartracks.scope', 'music-musicstore.scope', 'more_suggestions-ebay.scope', 'more_suggestions-ubuntushop.scope', 'more_suggestions-skimlinks.scope']"
Just a reminder though, I’m only guessing this is the issue because I’ve noted the time-delay sound changes before and tracked it down to monitor audio thinking the audio was gone when standby mode hits…followed by changing of audio when standby goes away (unfortunately on my multi-monitor setup on the desktop I can’t be sure that going to and from standby enforces the order of monitor startup…the software seems to depend on this, but should not). Every audio sink which gets added or removed from the list of audio devices has a chance to bump other audio sinks out of their expected place…this is one possible place for that to happen if it isn’t a bug in the kernel. I don’t even know for sure that the HDMI audio code would matter when no monitor is attached.