Hi Joe,
with the standard audio lib you could use
* biquad filters
* FIR filters
with a bit more fiddling you could use the audio lib queue object to obtain your audio blocks and do convolution filtering to give you the desired response.
The audio lib standard objects can be found in the Teensy audio lib GUI, the convolution filtering code examples can be found in the forum search function
Please note that the T4 does NOT have an DAC, it is not built into the chip. But for a bit lower quality audio you can use the MQS-output (do a forum search on that)
The T4 ADC has an effective number of bits of about 10 to 11.