Problems accessing Bing speech service with webwrite

I cannot seem to send audio data to Bing Speech service with the webwrite command. Sending a uint8 array obtained by reading in a wav file (i.e., including it as the data element in the webwrite statement) does not work . I have successfully used uint8 arrays in calls to the service with other scripting languages. Also, when I try setting the MediaType weboption to 'audio/PCMA', I get an error message "The media type 'audio/PCMA' is not permissible for numeric or logical data." What's that about? It seems to conflict with the documentation regarding data handling in webwrite.