Slower is not necessarily better. According to McTanza (original message here, seventh post, also mentioned here http://www-user.tu-chemnitz.de/~noe/LiteOn/C1-scans/index-eng.html), the LiteOn 48x can't work properly under 16x.
On the other hand, audio CDRs are made for 1-2x standalone burners. That makes them 2x certified !
Stories about burning at 2x or 4x for audiophiles have no scientific background. Not even blind listening tests.
I'm afraid you are the only one to be able to answer your own question.
40x LiteOn burner, 2x CDR, but made by Tayo Yuden... In your shoes, I'd try 24x first, then 8x if it doesn't work, then 1x, then if it still doesn't work, I would begin to worry about the CDR quality
Edit : by the way, if you can, test in Track At Once without finalizing the audio session. This way, you can test all speeds on one CDR, burning track after track.