I've never used TMRpcm…
Occasionally the audio plays, but the pitch is too high.
Make sure the sample rate meets the TMRpcm requirements (8kHz?).
A sample rate mismatch* will play-back at the wrong speed (with a corresponding pitch error). Also, make sure the mono-stereo is "correct".
A mono-file played-back as stereo would play at double-speed because it's reading twice as many samples (and it expects twice as many samples in the file).
On a computer, iPhone, or iPod, etc., the software/operating system reads the file header and it automatically adjusts to the correct playback sample rate, bit depth, and number of channels, and/or it re-samples as-required by the hardware and everything "just works". TMRpcm is not that "smart" and the Arduino is probably not fast enough for real-time resampling.