Recently, the Google has rolled out the July's Android Security Updates for the Nexus 5 smartphones a few days back. Now, as per the Google's AOSP Issue tracker, lots of users have reported a bug with the latest update MOB30P.
As per the report, some users are unable to adjust the volumes using the side keys and another user complained that the user at the other end can't use the speakerphone due to echo problem during calls.
Further, the search engine giant has acknowledged this issue and planning to release a bug fixing update soon.
Moreover, the Nexus Community Manager said on the Nexus Help Forum page, " It's been said a number of times in this thread the Google Development team is aware of an issue and a fix is in the works. In the meantime, it appears as if the OTA has stopped for those that did not receive it per several reports from other users.
As a temporary workaround, you can flash the MOB30M image from the Nexus Factory Images page using the steps in TechnoBill's guide found here: How To Flash A Nexus 5 (or 4) With A Factory Image
Added on edit: It's noted in the guide but it worth calling out that flashing a factory image will wipe all of the data on the device. Make a backup first and plan for that."