all compilers blacklisted or unavailable

[Jan Stary]

This is MacPorts 2.4.1 on MacOSX 10.6.8.
A build of audio/sox starts with the following warning:

$ sudo port install -d sox
Warning: All compilers are either blacklisted or unavailable; defaulting to first fallback option
Warning: All compilers are either blacklisted or unavailable; defaulting to first fallback option
--->  Computing dependencies for sox
--->  Fetching archive for sox
--->  Attempting to fetch sox-14.4.2_0.darwin_10.x86_64.tbz2 from https://packages.macports.org/sox
--->  Attempting to fetch sox-14.4.2_0.darwin_10.x86_64.tbz2 from http://nue.de.packages.macports.org/sox
--->  Attempting to fetch sox-14.4.2_0.darwin_10.x86_64.tbz2 from http://lil.fr.packages.macports.org/sox
--->  Fetching distfiles for sox
--->  Verifying checksums for sox
--->  Extracting sox
--->  Applying patches to sox
--->  Configuring sox
--->  Building sox
--->  Staging sox into destroot
--->  Installing sox @14.4.2_0
--->  Activating sox @14.4.2_0
--->  Cleaning sox
--->  Updating database of binaries
--->  Scanning binaries for linking errors
--->  No broken files found.

What "all compilers" are those? (I have Xcode 3.2.6)
Why are they blacklisted? Who blacklisted them?
Why are they unavailable? The gcc and clang from Xcode work just fine.
How do I get port(1) to print all this for me if -d doesn't?
WHy is the message printed twice?

	Jan