Sorry @mikeellwood and @netmouse, when I say ‘quite a few’, further investigation reveals about 5, one of which is Bore Da. One way is to search for ‘Welsh’ in Audible, and filter out what’s not Irvine Welsh et al!
We have:
Bore Da (Gwennan Evans)
Coed Y Brenin (Colin Jones)
Blasu (Manon Steffan Ros). Over 8 hours this one!
Cwm Gwrachod (Colin Jones again)
I Ble’r Aeth Heul Y Bore? (Eirug Wyn)
That’s about it. Sorry to get your hopes up.
But it’s a start. 
Wyt ti’n seren Netmouse, neu beth? Ie, seren wyt ti! Diolch!
Edit: Oh good. I see “Y Lôn wen” is there. That is also one of the relatively few books also available as a parallel (Welsh-English) text.
Diolch yn fawr iawn Colindale! Paid a phoenu Colin bach; that’s more than I’d expected! (and more than there used to be…well, I might have searched in the wrong way. Oh yes, IIRC, while there is a language search option in audible, it only works (or worked) for the major languages, and not Welsh.
(And while Irvine Welsh may be a perfectly nice guy, I have sadly grown to hate him because he always pops up in searches, where he is not wanted! (by me, anyway).
Diolch, @Colindale! I bought Bore Da a while back when I still had my Audible subscription, but haven’t really tried to listen to it yet. I checked, and all the books in your list are also available in the US Audible store
Well I just had a look at Audibles advanced search option and they only listed a choice of 12 possible languages so very little choice other than mainstream.
The first downloadable Welsh audiobook was only a year ago according to the following piece, so maybe things are chaning for the better?
http://cymraeg.gov.wales/news/index/llyfr-llafar?lang=en
but even though the link at the bottom of the piece is for Y Lolfa, i couldn’t easily find where the audiobooks are on there.
And what would the welsh for audiobook be? maybe thats where I’m going wrong with my search (audiobook cymraeg?, surely something different!)
Llyfr Sain would be a good bet.
Many thanks, will try that.
This was totally new to me (although it’s series 3), and just noticed in the schedules:
Cyfres o sgetsys wedi’u sgwennu gan wrandawyr a mynychwyr gweithdai Sgript Slam Radio Cymru.
Actorion: Richard Elfyn, Rhian Blythe, Sian Naomi a Sion Ifan.
(Google Translate):
Sketch series are written by listeners and attendees workshops Script Slam Radio.
Richard Elfyn is of course, Meirion in Byw Celwydd. The 2nd voice you hear sounds like David in Rownd a Rownd, but it can’t be, as “David” is played by Iestyn Arwel, who doesn’t appear to be in the cast of this radio programme.
I asked Carmarthenshire Libraries for an update. There are now Welsh language audio books available via Bolinda/Borrowbox. 38 of them. There are two not on loan at the moment. Better than nothing, which is what it was 18 months ago, but still not great.