no need for an attenuator with the Canon HV20 since you an adjust it's mic level.
When you record in HDV the audio recorded is 16 bit MPEG-1 Layer 2 (MP2), while in DV mode it records 16 or 12 bit PCM audio.
HDV video is four times the resolution of DV, but PCM is superior to MP2. MP3 audio (a compression of PCM) has a higher sound quality than MP2 because it has a greater scope of sub-bands that reduce the bit-rate while pumping up the clarity.
check:
http://www.camcorderinfo.com/content/Canon-HV20-Camcorder-Review/Audio--Playback--Connectivity.htm