Impedance of hands-free speaker
Lightweight headphones and earbuds usually have a nominal impedance of 32Ω.  A few tens of milliwatts should be enough to drive them properly.  Once you get into hundreds of milliwatts, the sound levels can become dangerous.

The efficiency of headphones and speakers varies considerably, so you'd need the specifications to work out how many milliwatts you'd need for a given volume level.

See also What wattage should a headphone amp be?