PowerShellを経由させてみるとか。
https://ascii.jp/elem/000/004/055/4055975/
例1
#include "hsp3dish.as"
text = "こんにちは HSP!"
cmd = "powershell -Command \"$x=New-Object -ComObject SAPI.SpVoice;$x.Speak('" + text + "');\""
exec cmd, 2
例2
#include "hsp3dish.as"
#include "hspext.as"
text = "こんにちは HSP!"
cmd = "powershell -Command \"$x=New-Object -ComObject SAPI.SpVoice;$x.Speak('" + text + "');\""
sdim ln, 4096
sdim buf, 32000
pipeexec buf, cmd, 1
if stat : dialog "実行できませんでした" : end
repeat
pipeget ln
if stat == 0 : break
await 10
loop
stop