In general you pass from the applescript to the next action whatever it needs using the "return" command at the end of the code. In your case though the automator action "Text to Audio File" doesn't accept a fileName variable so if you want that much control you need another method. Luckily that automator action can be replaced easily in the applescript code with a simple "say" command.
So create your automator service and receive the selected text. Then add an applescript action and use the following as the code. Then add an "Encode to MPEG audio" action.
For the applescript code just modify the voiceName and saveFolder variables with values of your choosing. The saveFolder path must end with a colon (:). Note that I use 26 instead of 30 because we add ".aif" to the end of the filename... to get a total of 30 characters.
on run {input, parameters}
set voiceName to "Jill"
set saveFolder to path to desktop as text
set selectedText to item 1 of input
if (length of selectedText) > 26 then
set fileName to text 1 thru 26 of selectedText
else
set fileName to selectedText
end if
set fileName to fileName & ".aif"
set filePath to saveFolder & fileName
say selectedText using voiceName saving to file filePath
return {POSIX path of filePath}
end run
If you need to determine your saveFolder use this to get the path. Run this code and copy/paste the result into the saveFolder variable above.
(choose folder) as text