Hold the queue in the caller

Instead of sleeping and then dropping the queue in `play_sound`, pass the
queue back to the caller for safekeeping.  Not pretty, but it makes the
program work as expected.

I think AudioQueue may just be broken.  I would expect to be able to call
`AudioQueue.queue_audio` repeatedly on the same object and have it play
the sounds in sequence, or immediately, if no sound is currently playing.
That's how it works on macOS.  But on Linux (with PulseAudio), only the
first `queue_audio` call plays anything, and subsequent calls are silent.
Several variations of calls to `.pause`, `.resume`, and `.clear`, didn't
improve matters.

So the fix is to make a new queue for each sound and make sure it isn't
dropped until that sound has finished playing.

src/bin/letters.rs

@@ -1,6 +1,5 @@
 use std::collections::HashMap;
-use std::time::Duration;
-use sdl2::{ttf::{self, Font}, pixels::Color, event::Event, keyboard::Mod, render::Canvas, video::Window, rect::{Rect, Point}, audio::{AudioSpecWAV, AudioSpecDesired}, Sdl};
+use sdl2::{ttf::{self, Font}, pixels::Color, event::Event, keyboard::Mod, render::Canvas, video::Window, rect::{Rect, Point}, audio::{AudioQueue, AudioSpecDesired, AudioSpecWAV}, Sdl};
 
 fn main() {
     let sdl = sdl2::init().unwrap();
@@ -15,6 +14,8 @@ fn main() {
     canvas.clear();
     canvas.present();
 
+    let mut queue: Option<AudioQueue<u8>> = None;
+
     for event in sdl.event_pump().unwrap().wait_iter() {
         match event {
             Event::Quit { timestamp: _ } => { break },
@@ -38,7 +39,7 @@ fn main() {
                         // display and play sound for any letter or number
                         if c.is_alphanumeric() {
                             draw_letter(&mut canvas, &font, c);
-                            play_sound(&sdl, sounds.get(&c).unwrap());
+                            queue = Some(play_sound(&sdl, sounds.get(&c).unwrap()));
                         }
                     }
                 }
@@ -47,7 +48,7 @@ fn main() {
         };
     }
 
-    println!("quit");
+    println!("quit: {}", queue.is_some());
 }
 
 fn draw_letter(canvas: &mut Canvas<Window>, font: &Font, c: char) {
@@ -85,7 +86,7 @@ fn init_audio() -> HashMap<char, AudioSpecWAV> {
     sounds
 }
 
-fn play_sound(sdl: &Sdl, sound: &AudioSpecWAV) {
+fn play_sound(sdl: &Sdl, sound: &AudioSpecWAV) -> AudioQueue<u8> {
     let desired = AudioSpecDesired{
         freq: Some(sound.freq),
         channels: Some(sound.channels),
@@ -95,6 +96,5 @@ fn play_sound(sdl: &Sdl, sound: &AudioSpecWAV) {
     let sound_queue = audio.open_queue(None, &desired).unwrap();
     sound_queue.queue_audio(sound.buffer()).unwrap();
     sound_queue.resume();
-    let audio_len = sound.buffer().len(); // bytes 
-    std::thread::sleep(Duration::from_millis(audio_len as u64 * 1000 / sound.freq as u64 / sound.channels as u64));
+    sound_queue
 }