Found out that startup.wav was made possible in the latest Clover build based off of https://github.com/Goldfish64/AudioPkg#BootChimeCfg
Install the latest Clover build. Install AudioDxe-64.efi option for UEFI and stick sound.wav inside the theme folder of your choice.
When you get to the Clover GUI, you need to select Options and select Startup Sound Output; select your audio output. You can test by selecting the F7 key and you can also control the volume in increments as 0010 up to 0100. Recommended setting is 0060. You'll hear a sound that resembles an electric guitar chord when doing the test, however the startup.wav or Apple chime will sound when the Clover gui loads.
I've seen some mixed results over at Insanely where some people have had the startup.wav initialize as the Apple logo displays; however I think that is only possible with the Goldfish64 option. There is also an option for PlayAsync in the config.cfg update in Clover where it should be selected. This can be found in Gui, under Theme.