Software to balance sound levels and quality for iPhone

You can use X Lossless Decoder(XLD) too:

X Lossless Decoder(XLD) is a tool for Mac OS X that is able to decode/convert/play various 'lossless' audio files. The supported audio files can be split into some tracks with cue sheet when decoding. It works on Mac OS X 10.4 and later.

because it's lossless it keeps the best sound quality.

Apple recommends using the following formats for your iTunes:

• Audio files (AAC, MP3) with appropriate file extensions (.m4a, .mp3). For best results during playback, Apple recommends converting MP3 files greater than 32 megabytes (MB) to AAC format. For AAC audio files, Apple recommends the following specifications:
• Audio Format: Baseline Low-Complexity Profile with AAC-LC (Music)
• Data Rate: Up to 160 Kbps
• Channels: Stereo in .m4v, .mp4, and .mov formats
• Output Sample Rate: 48 kHz
• Encoding Quality: Better

iTunes will not read the tags that MP3Gain uses to set the volume. iTunes stores Sound Check volume information in the iTunNORM tag.

If all of your music is in MP3 format, you could try qNormal.

However, if you've enabled iTunes Match and have a mix of MP3 and AAC files, you could try the commercial product beaTunes. It has a Windows version.

Both applications claim to calculate the appropriate volume using the open source ReplayGain algorithm that MP3Gain also uses.