Matching Buffer Formats
The Microsoft DirectSound mixer converts the data from each secondary sound buffer into the format of the primary buffer. This conversion is done on the fly as data is mixed into the primary buffer, and costs CPU cycles. You can eliminate this overhead by ensuring that your secondary buffers and primary buffer have the same format. Normally, this means setting the primary buffer format to the format of the WAV files used for data.
Because of the way DirectSound does format conversion, you only need to match the sample rate and number of channels. It does not matter if there is a difference in sample size (8-bit or 16-bit).