Audio output voltage range
Audio gear is genearally designed to have nominal levels of “-10dBV” (consumer) or “+4dBu” (professional).
-10dBV corresponds to 0.3Vrms and +4dBu to 1.2Vrms.
But these are merely “nominal” levels. A certain amount of headroom, generally 12dB or more, is added to allow for the loudest signals to not clip.
But these are for the synthesizer outputs that go into an amplifier or another piece of gear.
If you’re driving headphones or speakers directly that’s a whole different story.
Low impedance headphones and earbuds (8-32 Ohms) need only a small voltage, under 1Vrms max but a fair amount of current, up to 100mA.
High impedance  headphones (300-600 Ohms) need a high voltage (4-5V rms max) but only a few mA.