make sure that setDefaultsPTB does not overwrite audio defaults

because that's the whole point!

Author: Remi-Gau

src/utils/setDefaultsPTB.m

@@ -48,8 +48,8 @@
 
     if isfield(cfg, 'audio') && cfg.audio.do
 
-        cfg.audio.devIdx = [];
-        cfg.audio.playbackMode = 1;
+        fieldsToSet.audio.devIdx = [];
+        fieldsToSet.audio.playbackMode = 1;
 
         fieldsToSet.audio.fs = 44800;
         fieldsToSet.audio.channels = 2;