fix playing mono files on stereo output

Author: dhalbert

ports/atmel-samd/common-hal/audioio/AudioOut.c

@@ -405,7 +405,13 @@ void common_hal_audioio_audioout_play(audioio_audioout_obj_t *self,
     if (self->right_channel == &pin_PA02) {
         right_channel_reg = (uint32_t)&DAC->DATABUF[0].reg;
     }
-    if (right_channel_reg == left_channel_reg + 2 && audiosample_bits_per_sample(sample) == 16) {
+
+    size_t num_channels = audiosample_channel_count(sample);
+
+    if (num_channels == 2 &&
+        // Are DAC channels sequential?
+        left_channel_reg + 2 == right_channel_reg &&
+        audiosample_bits_per_sample(sample) == 16) {
         result = audio_dma_setup_playback(&self->left_dma, sample, loop, false, 0,
             false /* output unsigned */,
             left_channel_reg,
@@ -415,7 +421,11 @@ void common_hal_audioio_audioout_play(audioio_audioout_obj_t *self,
             false /* output unsigned */,
             left_channel_reg,
             left_channel_trigger);
-        if (right_channel_reg != 0 && result == AUDIO_DMA_OK) {
+        // Don't play back on right channel unless stereo.
+        // TODO possibility: Set up non-incrementing DMA on one channel so they use the same samples?
+        // Right now, playing back mono on both channels causes sample to play twice as fast.
+        if (num_channels == 2 &&
+            right_channel_reg != 0 && result == AUDIO_DMA_OK) {
             result = audio_dma_setup_playback(&self->right_dma, sample, loop, true, 1,
                 false /* output unsigned */,
                 right_channel_reg,