process_fn needs filename as return value

app.py

@@ -20,6 +20,7 @@ def process_fn(input_audio_path, seed, guidance_scale, num_inference_steps, num_
     
     sf.write("./output.wav", waveform[0, 0], samplerate=16000)
     #save_wave(waveform, "./", name="output.wav")
+    return "./output.wav"
 
 
 card = ModelCard(