mplayer says :

Playing piste_55.flac.
Audio file file format detected.
==========================================================================
Opening audio decoder: [ffmpeg] FFmpeg/libavcodec audio decoders
AUDIO: 44100 Hz, 2 ch, s16le, 235.2 kbit/16.67% (ratio: 29400->176400)
Selected audio codec: [ffflac] afm: ffmpeg (FFmpeg FLAC audio decoder)
==========================================================================
AO: [alsa] 48000Hz 2ch s16le (2 bytes per sample)
Video: no video
Starting playback...

my mpd.conf : 

eroux@norgz-laptop:~/musique/tests$ grep -v ^# /etc/mpd.conf 

music_directory		"/home/eroux/musique"
playlist_directory	"/var/lib/mpd/playlists"
db_file			"/var/lib/mpd/tag_cache"
log_file		"/var/log/mpd/mpd.log"
error_file		"/var/log/mpd/errors.log"
pid_file		"/var/run/mpd/pid"

state_file		"/var/lib/mpd/state"

user                            "mpd"
bind_to_address                 "localhost"
port                            "6600"
log_level                       "default"

audio_output {
        type                    "alsa"
        name                    "My ALSA Device"
        format                  "44100:16:2" # optional
}
audio_output_format             "44100:16:2"

mixer_type                      "alsa"
mixer_device                    "default"
mixer_control                   "PCM"


max_connections                 "2"

filesystem_charset              "UTF-8"
id3v1_encoding                  "UTF-8"

metadata_to_use                  "artist,album,title,track,name,genre,date,composer,performer,disc"