Some musics on websites have the sound volume so low that even with all sound settings at maximum levels it's hard to listen. Is there any mac software that can fast and simple increase the volume without degrading quality?Yes, you can free try MP3 Gain.

MP3 Gain for mac is a simple tool designed specifically to increase and normalize the volume of audio(mp3, aac, wma, etc) files. It automatically normalize the volume of audio files and increase to the uniform volume target without degrading quality. The target volume is in accordance with the EBU R128 loudness standardization specification. It performs statistical analysis to determine how loud the file actually sounds to the human ear. 

Key feature:

• Support many audio formats(mp3, wma, wav, flac, m4a, ac3, mp2, ogg)
• Comply with the EBU R128 loudness standardization specification
• Support batch increase the volume