From Music Player Daemon Community Wiki

[edit] Basic Troubleshooting

MPD does crossfading by using the buffer. The amount of buffer used for crossfading is the size of the buffer minus the amount of space reserved for buffer_before_play. By default (2MB buffer and 25% buffer_before_play) this is approximately 9 seconds of crossfading for 44.1 khz, 16-bit, stereo audio. If you have the buffer_before_play set to 100%, there will be no crossfading. If you'd like more crossfading than the default buffer settings permit, increase the buffer_size and/or decrease buffer_before_play in your config file (a sample config file is included with the MPD source and in the man page). Note that setting the crossfade amount only sets the maximum amount of crossfading, it does not guarantee that much crossfading will be performed.