In the output there’s a diode, which will drop the voltage some, and then there’s a resistor and LED after the 1k series resistor, which will act as a voltage divider. Depends on the LED but a quick LTSpice check gives me 5.99 V at the output with 10 V going into the diode, so what you’re seeing sounds roughly right.
That’s indeed a rather low amplitude. I think most envelopes are 8 to 10 V at maximum. Removing the LED or moving it (and its resistor) before the output series resistor would give you something around 9 V, probably.