Chances are the latter is more likely, at least according to a recent tweet by HTC Ireland who seemed to suggest in a video that the HTC One M9 could come with bigger/louder BoomSound speakers. Assuming the renderings are accurate, it doesn’t seem like they are physically bigger but perhaps the technology has been upgraded.
Previously with the HTC One M8, the company partnered with Harman/Kardon for the sound technology, and there were rumors about HTC possibly working with Bose, but unfortunately it seems that those rumors were quickly shot down by Bose themselves. Apart from teasing its speaker size, not much else about the upcoming handset is known to us save for the rumors.
In any case HTC will be revealing all officially come 1st of March at their MWC 2015 event, so if you’d like to see what HTC has in store for us with the HTC One M9, do check back then.