The incremental security updates that Android gets are generally regarded as a good thing, but with every software update comes the chance that something could get borked. That's doubly true with faster updates, and it seems to be the case with the latest Marshmallow release for the Nexus 5. According to our own readers, commenters on XDA and Reddit, and not least users on the official AOSP issue tracker, the original Nexus 5 is encountering some serious problems with volume control on the latest build, MOB30P.
The specific nature of the problem seems to vary quite a bit. Some users say that the volume can't be adjusted during a call, others say that it can't be set to zero in any app, others say that phone volume in general is just low to the point of being almost inaudible, among other similar problems. At least a few people at Google are aware of the issue, as company accounts have replied to one of the AOSP issue tracker threads.
While you wait for a fix, the easiest solution is probably just flashing your phone back to the previous Android 6.0.1 build (MOB30M) and waiting for the next monthly patch. Some users say that simply flashing an older kernel, or any of the many custom kernels available from ROM developers, fixes the problem. Your mileage may vary, of course. In the meantime join in the issue tracker threads or submit a bug report.
Over on the dedicated Nexus 5 subreddit, a verified Google community manager says that the company's development team has identified and fixed the volume problem. A fix will be rolling out "soon," which probably means sometime next week - the comment was posted on Friday.