fix(fw): compilation without AUDIO (#5022)

radio/src/mixer.cpp

@@ -1126,7 +1126,9 @@ void evalMixes(uint8_t tick10ms)
   // must be done after mixing because some functions use the inputs/channels values
   // must be done before limits because of the applyLimit function: it checks for safety switches which would be not initialized otherwise
   if (tick10ms) {
+#if defined(AUDIO)
     requiredSpeakerVolume = g_eeGeneral.speakerVolume + VOLUME_LEVEL_DEF;
+#endif
   
     requiredBacklightBright = g_eeGeneral.getBrightness();