How much is impedance of a typical hands-free speaker and how much power is needed to properly drive it?
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.