Riferimento: problema audio
guarda che ho trovato sulle FAQ di displayport:
Q: I'd like to know if I use a DP --> HDMI adaptor to connect my computer to my TV, will I hear the sound with my TV or my computer?
A: DisplayPort does handle full HDMI signaling including sound but keep in mind that implementation of full HDMI signaling is an option the PC manufacturer must choose to enable. It is recommended that you check with the PC manufacturer prior to making a purchase and verify that the PC supports DP++ capability with HDMI. DisplayPort adaptors simply passes through the DVI or HDMI signal.
in soldoni devi contattare Dell e chiedergli se nella loro implementazione trasmettono anche l'audio