How loud are MP3 players and other personal listening devices?

Though MP3 players and other personal listening devices have become smaller over the years, they still pack a punch. Some devices have volume outputs of 100 - 107 decibels and higher(1).

Answer Extra: 

As a rule of thumb, you can listen at 80% of a personal listening device’s maximum volume safely with earbuds or headphones for 90 minutes per day without the risk of noise-induced hearing loss. If you listen at a lower volume, you can listen for longer; for higher volumes, less time.

In 2009, the European Commission has recommended that MP3 players and other personal listening devices be limited to a default maximum volume setting of 80 dBA, a noise level comparable to a hair dryer or commuter noise. Though it would not limit the maximum output volume of the device, it would provide starting point for safe listening and provide the user with a warning that listening at too high volume over extended periods of time could cause noise-induced hearing loss.

source: 
Source: (1) Keith, Stephen E., David S. Michaud, and Vincent Chiu. “Evaluating the maximum playback sound levels from portable digital audio players.” Journal of the Acoustical Society of America, June 2008. http://www.ncbi.nlm.nih.gov/pubmed/18537374 (2) http://www.techradar.com/blogs/article/european-commission-orders-volume-down-on-mp3-players-640699