The problem — i.e. no sound — reared itself in the newish 2.6.24-1 kernel that recently rolled into Debian Lenny. It took me awhile before I figured out that booting the previous kernel — 2.6.22-3 — brought sound back.
Read it here
I did another Lenny install today on my test box, on which I did the “standard” install, added X and Fvwm (the window manager that comes standard with OpenBSD and is growing on me) and a few apps — Geany, Ted, Dillo, Lynx, Iceweasel, sudo and xterm (yep, a “standard” install doesn't include sudo and xterm).