[Milhouse]

Is it as simple as this to activate alsa (and external USB sound !!) on Openelec 4.0 beta??

Not OpenELEC Beta, but if you're building your own custom OpenELEC from source, then yes it's that simple. Since this is a compilation option it's not something you can enable on an already built system such as OpenELEC 4.0 Beta - either a system is built with ALSA support, or without (mine built with, stock/official built without).

[For my project I try to get pycurl and RPi.GPIO working. Compilation on 4.0 beta works. But I don't know which way to go now - enable alsa (and this is my main interest) on the standard or are there ways to inject to code into your builds?

There are ways and means of adding "third party" libraries to OpenELEC (either my custom build, or stock/official) - see here which adds a third party librtmp to override the built-in version. Not sure if this (or a similar solution) can be made to work for additional Python libraries, but might be worth investigating - certainly a lot easier than creating yet another custom build just for a couple of Python libraries.