pacman -Qi results:
Name : ardour
Version : 2.0beta11-2
Groups : None
Packager : Arch Linux (http://www.archlinux.org)
URL : http://ardour.org
License : None
Architecture : i686
Size : 18208759
Build Date : Tue Feb 6 05:12:46 2007 UTC
Install Date : Thu Mar 22 02:07:50 2007 UTC
Install Script : Yes
Reason: : explicitly installed
Provides : None
Depends On : jack-audio-connection-kit ladspa libart-lgpl libgnomecanvas
liblo liblrdf libsamplerate libusb libxslt
Required By : None
Conflicts With : None
Description : A multichannel hard disk recorder and digital audio workstation

This happened after updating flac - it still links to the old flac version. just re-compiling ardour should help.

Someone in the forums suggested I symlink libFLAC.so libFLAC.so.7, which did the trick :)

Well, the issue is a little worse as I suppose something in the current state in ArchLinux prevents arour from build at all. I can neither build the beta11.1 nor the new beta12. Both barf on linking to flac, and also the configuration summary tries to install the package to /usr/local instaed of /usr which is explicietly given as $PREFIX.
My best guess so far is that there was a scons update that might screw it up, but I'm just fishing. Not having a lot of time on my hands lately I appreciate any help in that issue.
So, recompiling as it is at the moment does not work, sorry

@xenobrain: this might work, but it is dirty workaround and I wouldn't recommend that at all.

I'm currently working deeper on this and I found actually ardour does not build with flac-1.1.4 - it does, however, with flac-1.1.2, and since flac-1.1.2 (both, beta11.1 and beta12) installs libFLAC.so.7 (while flac-1.1.4 uses libFLAC.so.8) these libs would coexist well. So maybe a solution would be to offer another package libflac-1.1.2 (based on flac-1.1.2-3 but containing the libs without symlinking libFLAC.so) and depend on that. But we still would need to hack the SConstruct file to use the headers from flac-1.1.2 which cannot be installed into the system.

I continue with this and will let you know, when I found a solution (instead of a dirty trick)

Thanks for digging into this, I will try to find out some news from the ardour front (ie, when they intend to move to flac1.1.4). As they are usually ahead of the game this prolly is a matter of a new beta release. I'll let you know if this works. For the time being it might be a solution to release a package that actually contains libFLAC.so.7 but I leave that to you what ever seems the easiest solution you can come up with. My best guess really is it won't be necessary for long.

Oh, and if things are too much of a hassle we can conside to return to the stable ardour release. After all we are building betas here.

Second edit:
I just found that: it might help:
http://www.arcknowledge.com/gmane.comp.audio.ardour.devel/2006-12/msg00027.html

Ok, here we go... Finally it builds fine.

Some words about the PKGBUILD attached:

1. I have *not* disabled SSE in flac-1.1.2 (while it is done in the archlinux PKGBUILD of flac-1.1.2-3) since the way ardour is built (even in the last package) uses SSE anyway, so why shouldn't ardour use a flac that takes advantage of SSE too. I know, not all i686 actually provide SSE, but I doubt ardour would run on these machines even when built without any SSE ;)

2. ardour comes with a lot of special ardour-versions of known libraries putting them in its own library directory, so I followed this scheme and have put the libFLAC.so.7 there, too. No additional package needed.

3. I have not included your forced usage of the CFLAGS replacing ardours -O3 -- this may break PKGBUILD guideline, I'm not sure, we do not have these rules in gnuffy, and I think ardour really takes advantage of the flags the ardour-team gave to it. Of course, you may change this back if you want to, it's not the reason why it didn't build.

4. I also have not included the additional install *.desktop file and icon. Actually ardour comes with its own icons in /usr/share/ardour2/icons/ardour_icon* in several sizes. Of course, you might add it again, I've just put the PKGBUILD here in the same way I publish it for gnuffy.

5. The configuration summary about the destination /usr/local is misleading. It is not the configuration summary of ardour, but instead it is the summary of the included version of libsndfile (which actually caused the flac-trouble) - but ardour does not really use this path for installation - it honours the DESTDIR given to scons.
This means line 36 of the PKGBUILD fixes the _output_ only, but you can safely leave this "--prefix=/usr" patch away.
The only reason I've put it in, is to prevent confusion on user side, who might press ctrl-c in panic, when he reads something about /usr/local running over the screen.

This is beta software, yes. but pretty stable beta software. I would refuse to return to the gtk1 version of ardour :)

Enjoy and continue making this world a better place, thank you :)

The current pkg on repository doesn't contain the libFLAC.so.7 in question but the PKGBUILD builds and includes this in the pkg if built locally. Probably a rebuild is necessary.

The version on the server is fine and contains the libflac file ?!?!

You're right, probably I just got an outdated mirror