Change default audio smoothing threshold to 50ms

70 milliseconds is a bit too high for the default audio timestamp
smoothing threshold.  The full range of error thus becomes 140
milliseconds, which is a bit more than necessary to worry about.  For
the time being, I feel it may be worth it to try 50 milliseconds.

libobs/media-io/media-io-defs.h

@@ -21,4 +21,4 @@
 
 /* time threshold in nanoseconds to ensure audio timing is as seamless as
  * possible */
-#define TS_SMOOTHING_THRESHOLD 70000000ULL
+#define TS_SMOOTHING_THRESHOLD 50000000ULL