There are 3 types of DVDA - pure audio titles (no graphical content and no Video_TS), Audio-Only titles (no video_TS) and so-called "universal" titles (with Video_TS).
1 - The Audio_TS is indeed where the files for DVDA reside, but with image burning applications you need to be certain that it will make a playable disc, and not simply a data disc with an Audio_TS folder on it. When you add in a Video_TS, it gets more confusing because of the specs and the way players work. I'll come back to this in a minute.
2 - The DVD-A world does work in the same way, but with added steps if there is to be a Video_TS. these titles are really 2 discs on one piece of plastic, as both title sets (Audio_TS & Video_TS) need to be authored in different applications, with the resulting Video_TS needing to be imported into the DVD-A authoring software.
Discwelder is an abstraction layer tool, meaning most of the interesting stuff is locked away & inaccessible. It does not create an Audio_TS folder on your HDD - it creates an image file instead (.dim) which can safely be renamed as an ISO file & then you can extract the Audio_TS from there. You cannot take a pre existing Audio_TS folder into DiscWelder, as far as I am aware - I might be mistaken here I am fairly certain this is correct.
You can take an existing Video_TS folder and merge it into a discwelder chrome project though, but to satisfy specs you should "point" all VTS titles from the discwelder project window.
Next up in the universal stakes is discwelder chrome (discontinued for some bizarre reason). This will allow importation & linking into the Video_TS in a limited way (if there is a VTS title with 2 audio streams, it is not possible to point the stream you want from the Audio Manager. It defaults to stream 1, and you must use the player remote to reset it).
Additionally, discwelder chrome really does assume that you know what you are doing if you want to create custom menus. Generated screens work okay though, but you may find trouble in making certain links work unless you know the specs & the limitations.
As we have arrived at the limitations, perhaps it is time to talk about how to create a Video_TS for use in DVD-A discs. There is one large, golden rule - you cannot use abstraction layer tools to do this as they will usually create dummy VMGM & VTSM menus, and this is out of spec. Recommended applications are either (on PC) Sonic's Scenarist (but with issues, as we will discuss) or Media Chance Labs DVD-Lab Pro - which also seems to be a dead product, sadly. On the Mac, Sonic's DVD Creator is still the gold standard and has no known issues - I would actually buy a mac just to run this if I could find a copy for sale.
DVD-Lab pro rules the roost for me - despite the fact it has some nasty bugs for the unwary - as it allows you to turn off the abstraction layer at compile. This means you need to write all the code manually or it won't work, but this is actually not too hard to do. It also allows something Scenarist cannot do - creation of slideshow titles where you can use a single image for each track and set track markers in the stream. With Scenarist, this cannot be done & if the music is segued or a live album, you must use low bitrate MPEG-2 streams. DVD-Lab's issues are the 2 big bugs....
A - you simply cannot create a title with more than at most 6 VTS or it will fail, and
B - if creating an Audio-Only title instead of a slideshow, you get gapping between tracks. It sets the cells as non seamless, meaning that despite the audio being one contiguous stream, it sets flags for non seamless playback and some players will then empty the buffers instead of continuing playback. This can be fixed in PGCEdit though, which is donationware.
Finally in the hybrid/universal stakes, we have Sonic's DVD-Audio Creator. This is the daddy. There is also a version by MEI, who leased it to Sonic for a while but then took it back.
This is full featured and there is nothing you cannot do with it. the power of dummy menus cannot be overstated, as it gives you places to hang code in the Pre Command areas to determine correct functionality in complex titles. Awesome, and I have never regretted buying it - but at around £2000 for a second user copy (if you can find one) it's not cheap.
It also only runs on Windows 2000/XP.
Now it's time to talk about players & zones.
With an audio-capable player, when you load a disc the first thing it will look for is a valid Audio_TS.IFO file in the Audio_TS folder. If it finds one, it will load the AMG (Audio ManaGer) of the disc and boot to the main menu. You should now have access to all content - including Video_TS content - on a correctly authored disc.
Failing this, it will decide this is not a DVD-A, and look for a valid Video_TS.IFO file in the Video_TS folder.
Failing that, it will decide it's not a DVD at all, and look for a TOC to see if it is a CD - this can be any form of CD supported by the player, and this varies.
Failing that, it will look for a ROM folder, although not all players can do this.
If it cannot find any of the above, it will decide it cannot load the disc.
I hope this helps, and please, please just shout if I did not explain any of this very well - I have an unfortunate tendency to waffle on......