AWSTranscribeExample.cls

public class AWSTranscribeExample extends AWS {
    public override void init() {
        endpoint = new Url('https://transcribe.us-west-2.amazonaws.com/');
        resource = '/';
        region = 'us-west-2';
        service = 'transcribe';
        accessKey = 'my-key-here';
        method = HttpMethod.XPOST;
        payload = Blob.valueOf('{"LanguageCode": "en-US","Media": {"MediaFileUri": "S3 audio file link"},"MediaFormat": "mp4","MediaSampleRateHertz": 44100,"TranscriptionJobName": "SampleRestJob7"}');
        setHeader('x-amz-target','Transcribe.StartTranscriptionJob');
        setHeader('content-type', 'application/x-amz-json-1.1');
        createSigningKey('my-secret-key-here');
   }
   public String executeCallout() {
       return sendRequest().getBody();
   }
}
// See https://gist.github.com/brianmfear/92cf05807ac4becbd21f for AWS

@brianmfear I'm getting below error when trying this code
<Error><Code>MalformedXML</Code><Message>The XML you provided was not well-formed or did not validate against our published schema</Message><RequestId>XXXXXX</RequestId><HostId>XXXX+XXXX=</HostId></Error>