Got an email from SanDisk technical support:
Thank you for emailing SanDisk Technical Support. It is our goal to make sure you have all the resources you need to get the most from your product.
Please note that playing mono OGG files is not fully supported on your Sansa Clip +. This means that you may have to convert the tracks to stereo so that you can listen to them normally. Please note that this may require the use of a third party application. Unfortunately, we will be unable to recommend a particular software but you may search online for applications that you may try for the said purpose.
However, we truly appreciate your efforts in bringing this to our attention and it has been duly noted. You may rest assured that we will be forwarding your observation to the appropriate department for consideration for future firmware releases.