Instead of using Apple's QuickTime Player, I would suggest looking into another player, such as VLC Player, or MediaPlayer Classic HC, both of which are free, and both of which have a very full set of CODEC's included in them.
For the best possible Audio, it would be best to choose PCM/WAV as the Audio CODEC. This is almost totally universal, and unlike all MPEG Audio (including AC3), is uncompressed. The file will be a bit larger (Audio does not take up as much space, as does the Video), but should give better quality, and not require additional CODEC's for users to play the file w/ Audio. This might not be an option in all possible Export/Share schemes.
For either PCM/WAV, or AC3 Audio, this will be set from the Audio Tab in the Export/Share settings. Remember setting the Encoding for the Audio might not be available in all formats, so just check that Audio Tab in the settings dialog screen (usually under Advanced).
As for DD Audio, you can set the Audio Encoding to AC3, wich is Dolby Digital, but that option might not be available with all possible Export/Share formats, so it gets down to an "it depends." This will be stereo (2-channel) Audio.
PrE cannot Encode to DD 5.1 SS (6-channel), as it lacks that Encoder. Even PrPro needs the Minnetonka Audio SurCode Encoder plug-in (US$300 option) to do DD 5.1 SS Export.
Thank you, Bill. Great information here. So, I looked at several preset options. Can't figure out the rhyme or reason as to why some audio options are associated with some video ones. For example, the NTSC DVD Widescreen preset does allow you to select the better quality PCM audio. But wouldn't a DVD preset mean the video is compressed? I'm assuming it would, as the anticipated file size is about half what it is for that settting as it is for the various HD ones I've already used. All of the presets I've exported my project out with so far only allow DD and MPEG2 audio. And I'm assuming I should ignore any of the presets under Adobe Flash Player and AVCHD.
I'll get the VLC player and use that as my default from now on.
... the NTSC DVD Widescreen preset does allow you to select the better quality PCM audio. But wouldn't a DVD preset mean the video is compressed?
Yes, a DV Preset will compress to MPEG-2 (the DVD-spec. format/CODEC), and if you have HD material, it will also reduce that to SD Frame Sizes.
Each format has allowable Audio settings for that format, and as you have found, they are not constant, nor common across various formats. A few, like FLV are extremely limited, while others have many options. It just depends on the format.
I would agree with Bill here that you should use VLC player more, for viewing your videos, as this is a more common and free player and plays almost all kind of video files as it includes a large set of codecs installed with it.