I’ve found the issue tends to be the speaker size so as not to have to perform ‘surgery’ to the chassis. YouChoos do a really good range of different speakers. The 6mm sugar curve from my experience gives a good sound level although there are even smaller ones that work pretty well.
For an N Gauge 25 the length of the chip and speaker might be a bit tight but I have successfully used the Zimo 648 chips in these Farish locos even with space for the resistors for the LEDs.
I am certainly going to experiment with these new chips as it looks like they will save the extra hassle of gaming to do a resistor ‘array’ for the lights. The 6 outputs also will allow for independent directional and cab lighrs