A couple of const correctness improvements.

Public issue https://github.com/mackron/miniaudio/issues/584

miniaudio.h

@@ -4983,7 +4983,7 @@ MA_API ma_result ma_fader_init(const ma_fader_config* pConfig, ma_fader* pFader)
 MA_API ma_result ma_fader_process_pcm_frames(ma_fader* pFader, void* pFramesOut, const void* pFramesIn, ma_uint64 frameCount);
 MA_API void ma_fader_get_data_format(const ma_fader* pFader, ma_format* pFormat, ma_uint32* pChannels, ma_uint32* pSampleRate);
 MA_API void ma_fader_set_fade(ma_fader* pFader, float volumeBeg, float volumeEnd, ma_uint64 lengthInFrames);
-MA_API float ma_fader_get_current_volume(ma_fader* pFader);
+MA_API float ma_fader_get_current_volume(const ma_fader* pFader);
 
 
 
@@ -11124,7 +11124,7 @@ MA_API void ma_sound_set_directional_attenuation_factor(ma_sound* pSound, float
 MA_API float ma_sound_get_directional_attenuation_factor(const ma_sound* pSound);
 MA_API void ma_sound_set_fade_in_pcm_frames(ma_sound* pSound, float volumeBeg, float volumeEnd, ma_uint64 fadeLengthInFrames);
 MA_API void ma_sound_set_fade_in_milliseconds(ma_sound* pSound, float volumeBeg, float volumeEnd, ma_uint64 fadeLengthInMilliseconds);
-MA_API float ma_sound_get_current_fade_volume(ma_sound* pSound);
+MA_API float ma_sound_get_current_fade_volume(const ma_sound* pSound);
 MA_API void ma_sound_set_start_time_in_pcm_frames(ma_sound* pSound, ma_uint64 absoluteGlobalTimeInFrames);
 MA_API void ma_sound_set_start_time_in_milliseconds(ma_sound* pSound, ma_uint64 absoluteGlobalTimeInMilliseconds);
 MA_API void ma_sound_set_stop_time_in_pcm_frames(ma_sound* pSound, ma_uint64 absoluteGlobalTimeInFrames);
@@ -48493,7 +48493,7 @@ MA_API void ma_fader_set_fade(ma_fader* pFader, float volumeBeg, float volumeEnd
     pFader->cursorInFrames = 0; /* Reset cursor. */
 }
 
-MA_API float ma_fader_get_current_volume(ma_fader* pFader)
+MA_API float ma_fader_get_current_volume(const ma_fader* pFader)
 {
     if (pFader == NULL) {
         return 0.0f;
@@ -74737,7 +74737,7 @@ MA_API void ma_sound_set_fade_in_milliseconds(ma_sound* pSound, float volumeBeg,
     ma_sound_set_fade_in_pcm_frames(pSound, volumeBeg, volumeEnd, (fadeLengthInMilliseconds * pSound->engineNode.fader.config.sampleRate) / 1000);
 }
 
-MA_API float ma_sound_get_current_fade_volume(ma_sound* pSound)
+MA_API float ma_sound_get_current_fade_volume(const ma_sound* pSound)
 {
     if (pSound == NULL) {
         return MA_INVALID_ARGS;