Tested, patch, compile, run with latest rev 26702.
Specifics:
Install instructions for fluidsynth music driver: Copy patch file to openttd folder, patch with: patch -p0 -i svn-fluidsynth-music-driver.patch, compile openttd with: configure --with-fluidsynth to enable fluidsynth driver (fluidsynth-devel needed).
Then when starting OpenTTD use ./openttd -m fluidsynth to select the music driver.
Parameters for it are: driver can be alsa or jack or pulseaudio etc. (what fluidsynth supports), defaults now to pulseaudio. soundfont with path/to/soundfont.sf2 uses that soundfont file. Finally audio-period-size, specifies the audio.period-size (audio buffer size) for fluidsynth to prevent underruns. Defaults to 512. values are 64-4096. I've added this because when I've tested this driver, with the default period size (64 I think on Linux), I got clipping in the audio and distortion. Both with alsa and pulseaudio as drivers for fluidsynth. I did google a bit this problem, and that led to allowing to specify the audio.period-size. 1024 seems to be a OK value also.
So for example: ./openttd -m fluidsynth:audio-period-size=1024,driver=pulseaudio,soundfont=/usr/share/soundfonts/FluidR3_GM.sf2
I've tested this, compiled and ran on Fedora 20 x64 with fluidsynth 1.1.6 installed and the default FluidR3_GM.sf2 soundfont file. Works great, has volume control for the music in game. I've tested the modern music set, got some warnings on some songs about missing instruments or misconfigured ones, but playing OK.
Using this though, I've noticed openttd using about 2% CPU (gnome system-monitor) and ~220MB RAM, while with timidity got 0-1% CPU and ~80 MB RAM usage. (CPU is Q6600 4core).
So please test and report any issues. Credits for it should go to the original author found in the thread above. Maybe if this is OK it can be added to trunk.