You should find that get volume settings will return an object containing among other things the output volume and the alert volume. So for example you could do this to retrieve the entire object:
osascript -e 'get volume settings'
or rather maybe this to grab just the output volume (e.g. rather than the alert volume):
osascript -e 'set ovol to output volume of (get volume settings)'
... but note that not all audio devices will have direct software control over volume settings. For example your display audio should have control; however, a firewire or USB i/o board probably would not have those settings under software control (since they might be physical knobs). If the particular setting is not under the control of software then it will show up in the object returned from get volume settings as "missing value" or something like that.