[bdp24]

Baltic Birch ply gets used a lot for braces. Has anyone used solid Birch for braces? Home Depot sells it in 0.75" X 1.5" strips for about a buck a foot.

[jcotner]

I am a hardwood fan myself, but there are two things I can think of that
might matter.

Dimensional stability. Plywood doesn't move the way hardwood does with
changes in environment. If a brace moves much it could bow your cabinet.

Possible resonant issues. I know this is true for MDF and I think for a
lesser degree with plywood but in general a laminated or composite
material should contribute less to the "noise" of a speaker cabinet
than a hardwood element might.

I'm sure if I've got this wrong you'll see no shortage of corrections. 

[bdp24]

I think I read somewhere that hardwood moves against the grain, not with it. If so, solid Birch braces wood be stable in length (I'll be running them across the interior of the enclosures, from wall to wall, Bill Fitzmaurice style), and move in width. Or do I have it backwards?

[Hank]

I don't trust solid hardwood to NOT move.  That said, if you're talking about small corner brace pieces, you'd probably be OK.  The secret to solid hardwood is to use what furniture craftsmen do:  use kiln-dried lumber that's at an optimum moisture content and then seal it.  There's antique furniture made of hardwood that hasn't warped or moved much, but it was certainly originally chosen after being naturally or artificially dried.  BTW, I've done many MDF speaker cabinets for many years and have not used anything but MDF braces with no issues.  Have fun!