Update speech-synthesis-markup-voice.md

Author: sally-baolian

articles/ai-services/speech-service/speech-synthesis-markup-voice.md

@@ -445,7 +445,7 @@ The following table describes the usage of the `mstts:audioduration` element's a
 
 | Attribute | Description | Required or optional |
 | ---------- | ---------- | ---------- |
-| `value` | The requested duration of the output audio in either seconds, such as `2s`, or milliseconds, such as `2000ms`.<br/><br/>This value should be within `0.5` to `2` times the original audio without any other rate settings. For example, if the requested duration of your audio is `30s`, then the original audio must otherwise be between 15 and 60 seconds. If you set a value outside of these boundaries, the duration is set according to the respective minimum or maximum multiple.<br/><br/>Given your requested output audio duration, the Speech service adjusts the speaking rate accordingly. Use the [voice list](rest-text-to-speech.md#get-a-list-of-voices) API and check the `WordsPerMinute` attribute to find out the speaking rate of the neural voice that you're using. You can divide the number of words in your input text by the value of the `WordsPerMinute` attribute to get the approximate original output audio duration. The output audio sounds most natural when you set the audio duration closest to the estimated duration.| Required |
+| `value` | The requested duration of the output audio in either seconds, such as `2s`, or milliseconds, such as `2000ms`.<br/><br/>The maximum value for output audio duration is 300 seconds. This value should be within `0.5` to `2` times the original audio without any other rate settings. For example, if the requested duration of your audio is `30s`, then the original audio must otherwise be between 15 and 60 seconds. If you set a value outside of these boundaries, the duration is set according to the respective minimum or maximum multiple. For output audio longer than 300 seconds, first generate the original audio without any other rate settings, then calculate the rate to adjust using the prosody rate to achieve the desired duration. | Required |
 
 ### mstts audio duration examples