Here's a scenario... you buy a portable DVD player. It says 'MP3' on the box. You could be forgiven for thinking that you could put some MP3s on a DVD and play them in your new portable player, but the chances are you can't. (I haven't, by the way, bought a portable DVD player. This particular screed isn't inspired by being ripped off myself, or it'd contain a great deal more swearing.)
The situation would be better if you were buying a DVD player to plug into a TV — hundreds of models (particularly cheap brands such as Pioneer) reliably do DVD-MP3 now, with a growing number of manufacturers using those successful generic firmwares.
The reason you'd want to do this should be as obvious to player manufacturers as to consumers: you can get about fifty albums onto a DVD and not have to root around for multiple discs. It should be all the light traveller wants or needs: a portable player that can be carried to watch a DVD or browse your music library during a journey, then with an adapter at your destination.
There's a good article at EMediaLive from 2002:
The CD Writer: Compressed Audio on Writable DVD--Why Not?:
Phone calls to some of the companies involved confirmed my worst suspicions. Most seemed genuinely puzzled at the suggestion that anyone would want to play MP3 tracks from DVD discs.
Sadly things haven't improved much for portable players since Hugh wrote that. There's very little information available on which brands and models support the feature, and a lot of disinformation based on reports that 'confirm' the player can handle it because the reviewer doesn't understand the difference between CD and DVD discs.
So why the reluctance of portable manufacturers to recognise standard file systems on DVD? A couple of genuine reasons (with suggested solutions) do spring to mind...
The battery life of a portable with a screen is usually less than three hours. It's already standard when playing MP3 on CD to spin the disc only when reading data, but an optimal interface would default to switching the screen off after a few minutes under battery power, resuming when a key is pressed.
A data CD can only hold a couple of hundred songs at reasonable quality. If the player attempts to catalogue all MP3s on a disc, it will need larger buffers for dealing with the quantities that could be on a DVD. Another solution would be to read only a directory at a time and enforce a maximum number of files per directory — plus this would prevent cataloguing taking a long time, keeping the device responsive when in use.
i.e. The reason we don't have functional portable players is probably also because the components are fractionally more expensive and the Eastern producers haven't yet committed to a generic firmware/OS with the necessary features. It's easier to keep churning out current models without investing a little in software programming as long as not too many people complain.
I have some hope that the common Matsui (Dixons Store Group) players might offer DVD-MP3, but little inclination to buy one on the off-chance. My guess would be that models playing DivX (MPEG-4) video will have the necessary navigation and logic to support a DVD with MP3s on, assuming you can select which DivX AVIs you want to play rather than just cycle through them. (And assuming they can play DivX from DVD as well as from CD, which can't exactly be taken for granted.)
Another possibility is to take an ordinary portable DVD player and work around limitations through software. Audio DVD Creator is a commercial offering — though I'm sure you could do similar with freeware — that can put 45 hours of music onto a DVD as 192kbps AC3. This is produced as a normal video DVD with menus from which you select tracks or playlists, so if your player can handle normal DVD-R/DVD+R burnt discs it'll work. But it's considerably more awkward than putting MP3s into directories and burning those to a disc, because everything has to be encoded again, and you'd probably want to do this from the original discs rather than MP3-->AC3 to prevent additional loss of quality.
Still, I suppose it'd be convenient once it was set up, and it's something to bear in mind if you've bought a portable player only to realise that 'MP3' on the box of a DVD player doesn't actually mean it plays MP3s from DVD. Assuming you haven't already headed back to the shop to get a refund and/or insert the device rectally into a manager...
Because in all seriousness, if the DVD players you're selling say "plays MP3!" on the box, they should bloody well play MP3s from the type of disc they're sold to play.