It is difficult to read mathematical expressions in braille, whether or not in coordination with text to speech (TTS) synthesis. The researchers in this study investigated the effect of an intervention on the braille readers' performance in reading and comprehending mathematical expressions as well as the braille readers' ability to make well-founded choices for the use of the braille display or the TTS synthesizer. The intervention consisted of adjusted TTS settings in combination with a professional development (PD) course for mathematics teachers of braille readers. Nine experienced braille readers, aged 13-18 years, whose teachers participated in the course, underwent the intervention. A pre- and post-test and a pre- and post-interview were administered. For mathematical braille reading skills, the percentage of correct answers increased from 22% to 41%. For speech comprehension, the percentage of correct answers only slightly increased from 56% to 59%. The results indicated a small positive effect on the braille readers' performance in reading and comprehending mathematical expressions and an improvement in making well-founded choices for the use of braille and speech synthesis. The results suggest that improving mathematical braille reading skills is the key to better mathematical performance.