I don't have the answers to all your questions, but this will get the ball rolling:
To play any form of media on your PC, you need the appropriate audio & video decoders (codecs) installed on your machine. That camcorder records audio in Dolby Digital, so my guess is that your laptop doesn't have a suitable decoder installed to handle that audio format.
One option is to install "AC3Filter", which should decode the Dolby Digital audio track and hence give you sound when playing the files back in WMP.
Another option is to install a codec pack, such as the "K-Lite Codec Pack". This has a whole range of decoders (including AC3Filter) which it will install and set up for you, allowing your system to playback virtually any audio & video format out there - including the AVCHD format files created by your camcorder when shooting in HD.
AC3Filter Free-Codecs.com : K-Lite Codec Pack
What exactly are you trying to backup to disk - the mpeg files 'as is' straight from the camcorder, or are you trying to burn them in standard DVD video format so they will play through a standalone DVD player?