Using sox for accessibility purposes - strech and pitch sound effects audio quality

Dear sirs and madams,
Thank you very much for developing this great and powerful tool.
I'm trying to use it to play speech output in Linux.
It plays wav files, containing tts output, handled by Speech-Dispatcher.
I  now want to control  speechrate and pitch with sound effects of sox.
I found out, that the stretch Parameter works quite good for controlling
the rate,
But if I configure it to be very fast, the sound quality gets much lower.
Are there some effects, which can hold the sound quality near to the Input
level while increasing the speed?
Thank you very much.
Best greetings
Joshua

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