Say is one of the first things I install on a new system! I usually grab the files from OS2.04, but the 1.3 versions do have that more "classic" sound
Lots of options for what you're looking to do, although User-Startup is probably easiest. You can use Say from the command line without invoking the UI. Adding the line
Say booting complete
should do it. You might want to put a Run >NIL: in front of it so that the boot process doesn't pause while it's talking. You can include the usual Say switches for pitch/speed/etc. as command-line arguments.
If you want to get complicated, you could also invoke Say's -x switch to speak the contents of a text file (so that you can change what the machine says without having to edit the entire User-Startup). If you installed the files for the SPEAK: DOS device, you could also
Copy yourtextfile SPEAK:
for the same result.
Finally, instead of User-Startup, you could put the commands into a separate script, give it a Project icon (set the default tool to IconX, add the tooltype DONOTWAIT) and put that in WBStartup.