We had this experience with a customer and felt it worth sharing -- may save you some troubles later.
Background: The customer was having trouble playing files at 44kHz so he bumped them down to 22 -- then he had noticeable clicks on the front and back ends of the audio. We suggested he download and install the new firmware (1.72) as this has improved buffering and tends to help slow SD cards play 44kHz files. When he did that, the files would not play at all.
Problem: Metadata in the file. The WAV spec allows for the embedding of other (non-audio) information in the file called metadata. This is often used for copyright information, application information, etc. It's fine for PC apps that have gigabytes of memory and gigahertz of processing power, but metadata is a real menace to embedded players like the AP-16+ that the user needs to have "right now" response from.
In [unreleased] version 1.40 I updated the code to stop when it hits a metadata chunk because that data is usually appended to the end of a file. This weekend, however, we learned that's not always the case. The app used to edit the audio for this project actually appended metadata to the front and back ends of the file. Since the front end metadata followed the WAV header (which is what the AP-16+ uses to determine playback size), the new AP-16+ firmware thought it had a zero-length file and didn't play it.
Solution: We opened the file in Audacity, found the metadata, and removed it. We suggest you do this with all files just to ensure the file you're playing is pure audio. And if you're using MP3s for another player you should do the same thing. If you've ever noticed that an MP3 is slow to play, this is the MP3 decoder chip having to wade through the metadata (which can be quite big in an MP3) to find the audio. Remember, small players don't have the horsepower of your PC so you cannot compare performance on one platform to the other.
The attached screen shots show the process
0) The file looks fine: stereo, 22kHz
1) In Audacity, click on the File\Open Metadata Editor menu
2) No matter what you see, click the Clear button to remove any metadata from the file
Use File\Export to save the newly clean file. Again, do this process for WAVs for your AP-16+ and for MP3s if using other products.
Wow, I had no idea Audacity would do that that easily! Pretty cool, Jon!