lavfi: ensure audio frame durations match the sample count

libavfilter/avfilter.c

@@ -997,6 +997,14 @@ int ff_filter_frame(AVFilterLink *link, AVFrame *frame)
             av_log(link->dst, AV_LOG_ERROR, "Sample rate change is not supported\n");
             goto error;
         }
+
+        frame->duration = av_rescale_q(frame->nb_samples, (AVRational){ 1, frame->sample_rate },
+                                       link->time_base);
+#if FF_API_PKT_DURATION
+FF_DISABLE_DEPRECATION_WARNINGS
+        frame->pkt_duration = frame->duration;
+FF_ENABLE_DEPRECATION_WARNINGS
+#endif
     }
 
     link->frame_blocked_in = link->frame_wanted_out = 0;