When Sox proxy is used any error messages will be lost. Instead they'll look like this:

Could not generate preview

Unable to communicate with Speechoid. http://speechoid-sox-proxy-1:5000array ( 'postData' => array ( 'lang' => 'sv', 'voice' => 'stts_sv_nst-hsmm', 'input' => '', 'input_type' => 'ipa', ), )

Sox proxy is only used as a hack solution to handle faulty audio from MaryTTS. It may not be needed in the future.