Mobo audio is standard feature, there wouldn't be any significant cost savings if there was a choice.
You can have multiple audio devices in a Pc, and a graphics card is one. Just need to toggle which audio device you want to use in Windows.
What audio system are you planning to use with the graphics card?
Pc speakers generally consists with only rca/aux and optical.
Hdmi, if you're using it for both video and audio, the audio system would either be a monitor with speakers or an Audio Visual Receiver to play audio to external speakers passing video out via another Hdmi cable to the display.