Does my sound card support hardware volume control?
The system_profiler data type corresponding to audio device information is SPAudioDataType, so you can run:

system_profiler SPAudioDataType


But this won't yield much information about hardware volume control capability of your sound card.

A better way of checking for more specific hardware information is to use ioreg command. In order to check whether your sound card supports hardware volume control run the following and then check whether your sound card appears in the results:

ioreg -trc IOAudioLevelControl


My Cambridge Audio USB DAC that doesn't have support hardware volume control does not appear on this list, while my other DACs and sound cards do appear on it.

If you are using an external USB device, you can also run the following to make sure your device is recognized as an audio device:

ioreg -trc AppleUSBAudioEngine


If your device supports hardware volume control, you will see one or more IOAudioLevelControl entries under your audio device's entry.

But apart from all the command line mumbo jumbo you can as well check the OSX's volume control when using your sound card to see if it is disabled!