In order to use aRts plugin on FreeBSD you must to do the following:

1) Move xmms binary to xmms.bin:

cd %%PREFIX%%/bin && mv xmms xmms.bin

2) Write custom xmms script:

--- cut here ---
#!/bin/sh
LD_PRELOAD=/usr/lib/libstdc++.so exec %%PREFIX%%/bin/xmms.bin "$@"
--- cut here ---

and put it to %%PREFIX%%/bin

/fjoe