Here are two commands to open the audio and video OSD settings. There is also a command "audio booster" which is a macro. It will of course only work on "analog" audio. For digital passthrough there is no volume amplification.
The macro actually works pretty well. It opens the window, goes down, goes right 5 times (you might want to increase this) and then closes the window with "back". It all happens very quickly.
Originally I tried to use the json command "Gui.Activatewindow," which is what I usually use with Frodo, but discovered another xbmc bug (it actually opens the window, but freezes and then comes back as "can't connect to xbmc" thus introducing an unpleasant delay of almost a second). So instead I used the execbuiltin version of activatewindow, which is only available to us thanks to the VoxTools addon for xbmc.
<?xml version="1.0" encoding="utf-16"?>
<commandGroup open="False" name="XBMC OSD Settings" enabled="True" prefix="" priority="0" requiredProcess="" description="">
<command id="324" name="Audio settings" enabled="true" alwaysOn="False" confirm="False" requiredConfidence="0" loop="False" loopDelay="0" loopMax="0" description="">
<action>
<cmdType>XJson.ExecBuiltin</cmdType>
<cmdString>activatewindow(osdaudiosettings)</cmdString>
<cmdRepeat>1</cmdRepeat>
</action>
<phrase>Audio settings</phrase>
</command>
<command id="332" name="Audio Booster" enabled="true" alwaysOn="False" confirm="False" requiredConfidence="0" loop="False" loopDelay="0" loopMax="0" description="">
<action>
<cmdType>XJson.ExecBuiltin</cmdType>
<cmdString>activatewindow(osdaudiosettings)</cmdString>
<cmdRepeat>1</cmdRepeat>
</action>
<action>
<cmdType>XJson.Raw</cmdType>
<cmdString>Input.ExecuteAction&&"action":"home"</cmdString>
<cmdRepeat>1</cmdRepeat>
</action>
<action>
<cmdType>XJson.Raw</cmdType>
<cmdString>Input.ExecuteAction&&"action":"down"</cmdString>
<cmdRepeat>1</cmdRepeat>
</action>
<action>
<cmdType>XJson.Raw</cmdType>
<cmdString>Input.ExecuteAction&&"action":"right"</cmdString>
<cmdRepeat>5</cmdRepeat>
</action>
<action>
<cmdType>XJson.Raw</cmdType>
<cmdString>Input.ExecuteAction&&"action":"back"</cmdString>
<cmdRepeat>5</cmdRepeat>
</action>
<phrase>Audio Booster</phrase>
</command>
<command id="336" name="Video settings" enabled="true" alwaysOn="False" confirm="False" requiredConfidence="0" loop="False" loopDelay="0" loopMax="0" description="">
<action>
<cmdType>XJson.ExecBuiltin</cmdType>
<cmdString>ActivateWindow(osdvideosettings)</cmdString>
<cmdRepeat>1</cmdRepeat>
</action>
<phrase>Video settings</phrase>
</command>
</commandGroup>