change default audio driver to SDL

A Linux user (Eleuin) has reported menu lockups with the default OpenAL
(probably thread related) and no lockups with SDL.

A Windows user has reported popping noises with all audio drivers except
SDL (#178).

Change the audio driver default to SDL for the time being until these
issues can be looked into.

src/wx/opts.cpp

@@ -279,9 +279,9 @@ opts_t::opts_t()
 {
     frameSkip = -1;
 #ifdef __WXMSW__
-    audio_api = AUD_DIRECTSOUND;
+    audio_api = AUD_SDL;
 #elif !defined(NO_OAL) && (defined(__WXMAC__) || defined(__WXGTK__))
-    audio_api = AUD_OPENAL;
+    audio_api = AUD_SDL;
 #endif
 #ifndef NO_OGL
     render_method = RND_OPENGL;