That is all good DJ, I stand corrected.
But the reason I was sure about being right, is of course, that it worked perfectly ok with /usr/bin/say.
do shell script "say 'Hi ' & say 'Hello ' & say 'hey there ' 2>&1 >/dev/null &"
In general, your way is the way to go, in order to make it work.
Edit
And since the script just contained this line, I didn’t notice that the do shell script command was eventually waiting for completion delaying the execution of any later statements. 