I found the answer in someone else's answer to a different question I asked. In an Ask Me Anything with Peter Mayhew (Chewbacca), this exchange happened:
QUESTION: As for me, I think that Chewbacca deserved a [medal] after the first death star destruction. What was that last word he spoke in Episode IV? Did it have something to do with not getting a [medal]?
PETER MAYHEW'S ANSWER: No, actually, the last words Chewie spoke in Episode IV was to dismiss the troops and thank them for their service.
Clearly, he was indeed giving orders.
I asked my buddies - one is an officer, helicopter rescue pilot, and instructor in the U.S. Coast Guard, the other is a Sergeant in the U.S. Army - what they think.
The Coast Guard officer, who is also a Star Wars buff, says this (by the way, Capy is my dog):
Interesting...hadn't watched the scene in months, and if you'd asked me cold I would have told you my recollection of Chewie's roaring was as if you'd taken Capy to city hall to get the key to the city and she randomly barked. But watching it now it seems clear that Chewie is, in fact, calling out the commands.
The US Army Sergeant said:
Well I'm an NCO, not an Officer, but I would point out that the majority of drill and ceremony commands have a preparatory command followed by a command of execution: Forward_March, Column-Right_March, Company_Attention, etc. There also single word commands knows as combined commands: Rest, Fall-Out, Fall-In, etc.
Some notes from a military study guide may help:
"a. The preparatory command is the command that indicates movement. Pronounce each preparatory command with a rising inflection. The most desirable pitch, when beginning a preparatory command, is near the level of the natural speaking voice. A common fault with beginners is to start the preparatory command in a pitch so high that, after employing a rising inflection for the preparatory command, it is impossible to give the command of execution with clarity or without strain. A good rule to remember is to begin a command near the natural pitch of the voice."
"b. The command of execution is the command that indicates when a movement is to be executed. Give it in a sharper tone and in a slightly higher pitch than the last syllable of the preparatory command. It must be given with plenty of snap. The best way to develop a command voice is to practice."
"c. In combined commands, such as FALL IN and FALL OUT, the preparatory command and command of execution are combined. Give these commands without inflection and with the uniform high pitch and loudness of a normal command of execution."
So one could infer that the Wookiee is simply giving a combined command: "Clap." Which would preclude the use of a preparatory command and command of execution. However I would note the slight rise of inflection near the end of the first instance which may merely be an example of poor form.