Add volume() function to AudioInputUSB

teensy3/usb_audio.cpp

 uint16_t AudioInputUSB::incoming_count;
 uint8_t AudioInputUSB::receive_flag;
 
-struct usb_audio_features_struct AudioInputUSB::features = {0,0,FEATURE_MAX_VOLUME};
+struct usb_audio_features_struct AudioInputUSB::features = {0,0,FEATURE_MAX_VOLUME/2};
 
 #define DMABUFATTR __attribute__ ((section(".dmabuffers"), aligned (4)))
 uint16_t usb_audio_receive_buffer[AUDIO_RX_SIZE/2] DMABUFATTR;

teensy3/usb_audio.h

 	friend void usb_audio_receive_callback(unsigned int len);
 	friend int usb_audio_set_feature(void *stp, uint8_t *buf);
 	friend int usb_audio_get_feature(void *stp, uint8_t *data, uint32_t *datalen);
-
 	static struct usb_audio_features_struct features;
-
+	float volume(void) {
+		if (features.mute) return 0.0;
+		return (float)(features.volume) * (1.0 / (float)FEATURE_MAX_VOLUME);
+	}
 private:
 	static bool update_responsibility;
 	static audio_block_t *incoming_left;