Broadcast radio receiver in the cab

Plays internet radio streams (http, mp3) through a positional emitter in the cab, with analog tuning, inter-station noise and two speaker profiles. Stations are listed in fmradio.txt, cab controls are defined with fmradio entries in the .mmd file.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

audio/sound.h

@@ -139,6 +139,9 @@ public:
     // returns location of the sound source in simulation region space
     glm::dvec3
         location() const;
+    // returns location-based gain factor of the emitter for the current position of the listener
+    float
+        soundproofing_factor();
     // returns defined range of the sound
     void
         range( float const Range );