+/** @brief Get/set volume
+ * @param api Sound api or -1 for default
+ * @param left Left channel level, 0-100
+ * @param right Right channel level, 0-100
+ * @param set Set volume if non-0
+ * @return 0 on success, non-0 on error
+ *
+ * If getting the volume then @p left and @p right are filled in.
+ *
+ * If setting the volume then the target levels are read from @p left and
+ * @p right, and the actual level set is stored in them.
+ */
+int mixer_control(int api, int *left, int *right, int set) {