1. You can set a trigger to execute a sound file from the BI "Sounds' folder, or any folder you want. The default player will open and play the file. This is the simplest method. The downside is that the player will stay open, and, if your watching/listening to anything, it will be interrupted.
2. NirSoft has a free tool called 'NirCmd'. Actually, they have a lot of free utilities. Website is: NirSoft - freeware utilities: password recovery, system utilities, desktop utilities
NirCmd, among other things, can set/change/mute system volume, and, speak text or a text file.
Set volume to 100%: nircmd setsystemvolume 65535
Speak: nircmd speak text "Motion triggered at back door."
Set volume to 25%: nircmd setsystemvolume 16384
Easier to set this as bat files for each type of speaking alert you want, then just call the bat file like in #1 above.
I've been testing using VLC for the past 30 minutes. Seems to be working fine.
In 'Alerts', 'Run a program or execute a script'...
1. Select the VLC executable: C:\Program Files\VideoLAN\VLC\vlc
2. In parameters: -I dummy "C:\Program Files\Blue Iris 4\sounds\gentle-alarm.wav" vlc://quit
Upon Alert, VLC will play the sound file specified in an invisible window.