It is up to you to decide. I personally prefer ALSA with some additions like JACK, but that is only because I think that OSS is too old and PulseAudio is too buggy. There are different opinions though. Most programs will do just fine with bare ALSA and in Funtoo/Gentoo you are free to recompile the whole audio system the way you want. And as far as I know it is pretty much the way USE flags are being set by default by the Funtoo profiles system when you set it to be a desktop with all the multimedia bells and whistles. Also when you have just installed the system and no configuration is done yet, no audio support is present at this stage at all. So it is hard to tell what is the default and what is custom because pretty much every Funtoo/Gentoo installation is inherently custom.