Same with V6 and V7 HDV. Don't know why, either. My default project properties are HDV 1080 60i, with 48k audio. I have to remember to always check the audio if I change the project properties.
It is in any Vegas Version set to 44.1 kHz because this is what almost any soundcard does take without problems. It matters for monitoring only, so 44,1 is the most safe setting.