My Thinkpad T400 had very low speakers volume. Turns out there is a way to bump volume up using hardware modification.
AN12946A has selectable gain modes using SP GAIN 1 (pin 27) and SP GAIN 2 (pin 28). By default T400 uses +13dB mode (which is lowest possible).
We can easily switch to +19dB mode by lowering SP GAIN 1 (pin 27) logic level (see AN12946A datasheet) and leaving SP GAIN 2 (pin 28) high. SP GAIN 1 (pin 27) uses 10k resitor for pull up. We are lucky – pin 26 is GND, so the
Result – speakers volume is better (not great though). I had to use 100% volume before, now about 80-85%.