Thank you for you answer.
The problem is that the installation of lib32-portaudio needs lib32-jack, but it's impossible to install lib32-jack and keep jack2.
I've got a problem if I change lib32-jack to jack2 in the dependancies.
I didn't try to install with lib32-jack.
There is the compilation's error :
libtool: link: gcc -m32 -shared -fPIC -DPIC src/common/.libs/pa_allocation.o src/common/.libs/pa_converters.o src/common/.libs/pa_cpuload.o src/common/.libs/pa_dither.o src/common/.libs/pa_debugprint.o src/common/.libs/pa_front.o src/common/.libs/pa_process.o src/common/.libs/pa_stream.o src/common/.libs/pa_trace.o src/hostapi/skeleton/.libs/pa_hostapi_skeleton.o src/hostapi/alsa/.libs/pa_linux_alsa.o src/hostapi/jack/.libs/pa_jack.o src/common/.libs/pa_ringbuffer.o src/hostapi/oss/.libs/pa_unix_oss.o src/os/unix/.libs/pa_unix_hostapis.o src/os/unix/.libs/pa_unix_util.o -lrt -lasound -ljack -lm -lpthread -m32 -Wl,-O1 -Wl,--sort-common -Wl,--as-needed -Wl,-z -Wl,relro -Wl,-soname -Wl,libportaudio.so.2 -Wl,-version-script -Wl,lib/.libs/libportaudio.ver -o lib/.libs/libportaudio.so.2.0.0
/usr/bin/ld: skipping incompatible /usr/lib/gcc/x86_64-unknown-linux-gnu/4.7.1/../../../libjack.so when searching for -ljack
/usr/bin/ld: skipping incompatible /usr/lib/libjack.so when searching for -ljack
/usr/bin/ld: cannot find -ljack
collect2: error: ld returned 1 exit status
make: *** [lib/libportaudio.la] Erreur 1
The problem seems clear: it doesn't like the x86_64 version of jack2.
So, is it possible to install lib32-portaudio without causing conflicts with my installed version of jack2 ?
I'm a musician, I can't drop jack2.