convert the sample to be played into a the following format: mono, 8KHz, 8bit
This can be done using a tool like: r8brain (from voxengo)
The sampleplayer only uses the 4 Most significant bits of the sample (in case of playback using the Commodore's sound chip).
This means that the sample must be as loud as possible (experiment with this as it distortion is also a matter of taste).
.WAV files can be pretty large, so take the time for it to upload, make sure that it fit's onto the remaining space of the 8MByte Cassiopei's flash memory.

If the sample does not sound right, then check if it is MONO, check the sample rate and check the bit depth of the file you've uploaded into the Cassiopei.