Do CD-R's play on your player? If no, why not?

Sometimes it is a reflectivity issue. CD-Rs are only 70% as reflective (on average, not sure where I got his number) as regular CDs. CD-RWs are even worse as they are only 30% as reflective as regular CDs. Some transport optics just can't deal with the lower reflectivity of the CD-R discs. And I doubt audio vs. computer CD-R stock would make ANY difference (assuming near identical media types) in being able to play them.

usually when I can't play a disc in some random PC CDROM or a audio cd player,

1. I try a slower buring speed like 4x or 2x, barring that
2. I try a different Brand of CD-R disc, barring that
3. I try different color CD-R discs.

I can usually get te disc to play by empirically figuring out which color substrate the particular player seems to prefer (probably a reflectivity issue).

See the following link for a discussion on CD-R substrate color:

http://arstechnica.infopop.net/OpenTopic/page?a=tpc&s=50009562&f=24609792&m=1240973441&r=7460961541#7460961541

FWIW, My Marantz CC-47 plays CD-Rs just fine. Never an issue with me. I've used both el-cheapo $0.10 USD blanks and expensive stuff without issue. Maybe I've just been lucky? The Marantz CC-47 is a 5 disc carousel, BTW.

And as others have mentioned, both DVD players will NOT play CD-R discs.