In Options > Playback > Audio, you can choose output methods like ASIO, WaveOut, WASAPI (v13 only), etc.
If you configure any of those, you'll be able to pick a specific sound card.
This setting is zone-specific, so if you create multiple zones you can target multiple soundcards.
To do the same for videos, look in Options > Playback > Video > Directshow playback settings.