As documented on MSDN.

The SAPI.SpVoice object (referred to as "SpVoice ") can be used to convert text to speech, either for immediate playback, or as a media file. SpVoice can use any system text-to-speech voices installed from language packs (although it requires some tweaking on modern windows).

SpVoice := ComObjCreate("SAPI.SpVoice") ; Note: You can save this object, you shouldn't create a new one each time you want to use it
SpVoice.Volume := 100
SpVoice.Speak("Hello world!")

By default, SpVoice.Speak() expects XML formatted text, with extra commands for the text-to-speech engine embedded. This means that you need to pass it valid XML and that it will throw an exception when you pass invalid XML, which probably isn't what you want. To disable this (or enable other behavior), the Flags second parameter of SpVoice.Speak() can be any of the following values bitwise-OR'd together (| operator):

Flag Name Flag Value Meaning
SVSFlagsAsync 1 Return instantly, don't wait to finish speaking
SVSFPurgeBeforeSpeak 2 Cancel any previous speaking that might still be going
SVSFIsFilename 4 The "text" to speak is actually the name of a file whose contents should be spoken
SVSFIsXML 8 The "text" to speak is XML, and contains extra commands for the text-to-speech engine
SVSFPersistXML 16 Any XML commands in the text which modify state will persist through all following SpVoice.Speak() calls
SpVoice := ComObjCreate("SAPI.SpVoice")
Voices := SpVoice.GetVoices()
VoicesText := ""
loop, % Voices.Count() {
	VoicesText .= Voices.Item(A_Index - 1).GetDescription() "`n"
MsgBox, % VoicesText
SpVoice := ComObjCreate("SAPI.SpVoice")
Voice := ... ; Either loop over SpVoice.GetVoices() to pick a voice, or use an index into that list
SpVoice.Voice := Voice
MsgBox, % VoicesText

In newer windows versions after installing language packs the voices might not be available through this COM object. However, you can use the following trick to "enable" them.

Simply copy the registry keys for each new voice from HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Speech_OneCore\Voices\Tokens to HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Speech\Voices\Tokens (for 64 bit applications), and to HKEY_LOCAL_MACHINE\SOFTWARE\WOW6432Node\Microsoft\SPEECH\Voices\Tokens (for 32 bit applications).