You just call it apparently

stream.lua

@@ -27,7 +27,7 @@ for i = 1, buffer_size do
     buffer[i] = 0
 end
 
-local decoder = dfpwm.make_decoder()
+local decode = dfpwm.make_decoder()
 while true do
     local chunk = req.read(buffer_size)
     if not chunk then
@@ -39,7 +39,7 @@ while true do
     --    buffer[i] = string.byte(chunk, i) - 128
     --end
 
-    local buffer = decoder.decode(chunk)
+    local buffer = decode(chunk)
 
     while not speaker.playAudio(buffer) do
         os.pullEvent("speaker_audio_empty")