The documentation of QAudioDeviceInfo::deviceName() states:

Device names vary depending on platform/audio plugin being used.
They are a unique string identifiers for the audio device.

At least on OSX this appears not to be true. I have a USB skype phone that in the OS shows up as an output and 2 inputs; OSX System Preferences->Sound reports them as:

(Output)
free.2
(Input)
free.2: Microphone
free.2: External Line Connector

QAudioDeviceInfo::availableDevices() correctly reports an output named "free.2", but only finds a single input, also called "free.2", which turns out to be the microphone. The external line connector remains inaccessible through this API.

My interpretation of this is:
1. The skype phone reports it's name as "free.2" on all 3 interfaces, thus making the name non-unique.
2. OSX appears to have a mechanism for rendering interface names unique by appending additional disambiguating information ("Microphone", "External Line Connector") where necessary.
3. QAudioDeviceInfo lacks (access to) that mechanism.