Comment 1 for bug 213520

Checking the source for libswfdec, the problem appears to be that it tries to mmap the audio buffer, which I don't think works with the alsa pulseaudio driver. See swfdec_playback_alsa.c line 221:

if (snd_pcm_hw_params_set_access (ret, hw_params, SND_PCM_ACCESS_MMAP_INTERLEAVED) < 0) {
    g_printerr ("Failed setting access\n");
    goto fail;
}

OTOH, swfdec does have a pulseaudio driver, which can be selected by passing --with-audio=pa to configure. With this setting enabled, sound works fine.