Here’s a link to the ifixit Omaker M4 Motherboard Replacement guide.
It is useful as it shows how to open the speaker case so that you can get to the speaker driver.
I cannot find an Omaker replacement for the speaker, but looking at the images in the guide, the information stamped on the speaker shows that it is a 4 Ohm 3 Watt speaker.
If you remove the speaker from its mounts in the case and measure its’ diameter you can use this information in conjunction with the above power specifications to get a suitable replacement.
Here’s a link to just one supplier of 4 Ohm 3 Watt speakers, showing speakers with varying diameters, hopefully one of which will fit inside the case.
You may find other suppliers that suit you better. Just search for (insert diameter of speaker) 4 Ohm 3W speaker to get results