Modern smartphones are designed with speakers that can handle a wide range of audio volumes without sustaining damage. Manufacturers typically ensure that the maximum volume level is within the safe operational limits of the speakers. This means that even at the highest volume, the speakers should not be damaged under normal conditions.
However, while occasional use at maximum volume is unlikely to damage the speakers, consistently playing audio at such high levels can potentially shorten the lifespan of the speakers. This is due to the increased wear and tear on the speaker components when they are consistently pushed to their limits. At max volume, you may experience a decrease in audio quality, characterized by distortion or a muffled sound. This is not necessarily indicative of damage but rather the speaker struggling to reproduce sound clearly at high volumes.
Apple may also include software limitations to prevent the speakers from reaching a volume that would cause damage.
***TL;DR*** Speaker shouldn't be damaged unless consistently playing audio at max volume, but even then it is likely that Apple has implemented some software that would limit the potential for any issues.