Okay, so I have the same issue. Our Sansa Shaker (pink, 512mb) will skip mid-song to another song when using the built-in speaker. The unit also sometimes plays songs at half-speed (slow-motion sounding). I spoke with technical support and they narrowly-defined the issue to this:
Tech support summary:
The Sansa Shaker may skip if the song your playing has a heavy bass signal. Any song with heavy bass that is played through the built-in speaker may create excessive vibration, that causes the player to skip. The Tech Support agent says the vibrations cause the memory card to skip. A low battery will increase the likelihood of skipping.
The official workaround is to reduce the bass signal on all your affected mp3 files. Use a third party program with a multi-band equalizer (not provided by Sansa) to reduce the bass signal, then re-encode or save the newly modified mp3, and transfer the modified files back into your Shaker. Or simply listen with headphones, to disable the built-in speaker, and the unit will not skip.
There is no upgradeable firmware on the Sansa Shaker, so no software workaround is available, and there is no offer to replace/exchange the unit because support says the unit is not defective. The unit is designed for children, so excessive bass is not advisable.
Testing the Sansa Response:
I tested this response by reformatting my SD card. Then I uploaded two versions of a song that repeatedly skipped; the original, and a modified version that had all bass signals below 225hz reduced to (-) infinity using a 20 band graphic EQ filter in Sound Forge.
The original continued to skip mid-song, and since there were only two songs on the card, it would skip to the modified version with a reduced bass signal. The modified version played all the way through. When playing both songs through headphones (disabling the built-in speaker), both songs played in their entirety, w/o skipping.
I DO believe tech support defined the issue correctly. I DON"T believe this was an intended child-safety feature. This was a unforseeable flaw in the products design. If a childs hearing were at issue, then they wouldn’t allow the song to play in the headphones either, right? A loud song played at high volume in headphones will damage an ear just as effectively as an external speaker would. Its the decibel level that is damaging, not the type of speaker. This is just an excuse not to refund, or exchange.
Re-encoding your mp3 collection is impractical and time-consuming, not to mention difficult to someone who is not computer-saavy.
A simpler workaround for listening w/o headphones is to lower the volume when playing through the built-in speakers (reducing the vibration), or plugging the unit into computer desktop speakers (bypassing the built-in speaker). You can also place it on a soft cloth, wrap a loose rubberband around it, put a small thin piece of tissue in the battery compartment, or suspend it by a lanyard using the eyelet, to create a pseudo-vibration damper.
The unit playing at half-speed was never addressed.