About fourteen years ago, two friends of mine and I bought about $300 worth of oak from some guy who sawed it himself. My share is of varying lengths and widths, and is 3/4" thick. It has been sitting down my basement for all this time. I doubt if I will ever use it for anything else. The only problem with it is that it has always had a bit of an unpleasant smell to it.

Am I likely to get cracking or anything else problematical?

Do I need to use an oak filler? If so, what brand is good?

I'd recommend staying away from solid hardwood when building cabinets for speakers. Use a good void-free baltic birch for large cabinets or MDF for smaller ones and put on a finish layer of nice real wood veneer. The idea is to make the cabinet as non-resonant as possible. The amounts of different wood fibers in these composite wood producs tends to break up any resonances. So I'd suggest using materials such as those and adequate bracing to make your cabinet very solid. That's the way seven π cabinets should be built.

I might add that there is a small minority of speaker builders that do just the opposite. Some like the idea of having purposely thin panels, and using them to vibrate and add something to the sound. Their idea is to voice the speaker cabinet like how the body of a guitar is voiced.

To me, the two things are different, because one is a sound generating instrument whereas the other is a sound reproduction device. With the musical instrument, the performer is creating something and with the loudspeaker, the user is attempting to copy something already performed. Hopefully the loudspeaker gives a convincing illusion that the copy is the same as the original, and doesn't sound like it has added anything to it. But the point I wanted to make is that there are some speaker builders that voice the cabinet like the body of a stringed musical instrument.

Wayne