The resistor R9 regulates the speech speed. If R9 goes out of spec it can affect the speech speed, you can check the value with the schematic - I installed a variable resistor in place of R9, adjusting the resistor I could change the speech speed from very slow to very fast (which is fun to play around with) then adjusted somewhere inbetween at a point where the speech sounded normal, this is how you can get your speech how you want it to sound.

I have had two TMS5200 chips that sounded different to each other, trying both in the same board I found one definitely sounded noticeably slower than the other, so the TMS5200 chip itself can also be the cause, in this case, the variable resistor allows for speech speed adjustment on the slower chip.

That's an interesting point, I wonder if they degrade/change over time.

I bought 7 NOS TMS5200NL chips nearly 20 years ago & I tested them when I got them, all the same date codes, I think 3 or 4 out of the 7 appeared to be faulty with garbled speech, the rest were ok, I sent them back & they sent me replacements - I remember when I had 7 working chips they did all had the same pitch when tested in the same board.

None had any leg oxidization when I got them, but I noticed recently that most now have a bit. I decided to vac seal the 6 of these I still have in a tube to halt this - this may be a first!

Also, going back earlier to around 1997, before their were any sellers on the net for chips like this, I needed a TMS5200 & could not find one anywhere or even a used S&T board at that time, so I bought a TI99 speech synthesizer which are cheapish & utilize a TMS5200.

Interestingly when I pulled the unit apart it had a different part number for the chip - CD2501E. It is funtionally identical to the TMS5200, just a different number. Works perfectly. That chip has also oxidised. No NOS CD2501E chips can be found anywhere.

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