1    [sal-voh]  

1. a simultaneous or successive discharge of artillery, bombs,etc.
2. a round of fire given as a salute.
3. a round of cheers or applause.
I had always heard this in the context of an argument or debate. As in:
 
“That was an excellent point! Senator Blowhard has fired the first salvo in this round.”
So, to my mind a salvo is the signal of launch. Not just in the strictest sense of artillery, but also of verbal barbs meant to incite a response.
I have also, perhaps incorrectly, used it in the sense of a greeting or hailing someone from afar. For some reason it brings to mind trumpets: a panoply along with that salvo, if you please!