You should be able to change the target register of the speaker library to write to the port set register instead of writing to the DAC value port. Hints on how to do this would be to look at which registers are used in the GPIO implementation for the STM32F2.
Depending on what the waveform you’re trying to generate looks like you may need 2 DMA channels: one copying to the port set register (BSRRL) an array where the 1’s mean turn on the pin and one copying to the port reset register (BSRRH) an array where the 1’s mean turn off the pin. Does that make sense?