Many of us N5 users have upgraded to Android 5.0 and have been running it on our devices since the first developer test build was released (months ago) and now even many more have upgraded to the official 5.0 LRX210 release.
With any major OS upgrade there will be some bugs and issues... These will usually get ironed out in the future builds/releases.
For instance some apps will work just fine, while others won't because they haven't been upgraded to support 5.0 just yet and yes since it's a new OS, something will be different and may have changed since the previous OS version.
While I do seem to somewhat agree with you that since the 5.0 update that the sound quality or lack of loudness does seem quieter and less powerful on 5.0.
One thing to consider is that the N5 has 2 speakers on the bottom of the device.
The left speaker is an audio speaker while the right speaker is a microphone, so music doesn't play out of both speakers.
With that being said... Are you rooted?
If so...
There is a very nice sound enhancer/mod that make's your N5 basically surround sound and it is extremely easy to install and/or uninstall -
http://forum.xda-developers.com/note-edge/development/mod-sm-915t-surround-sound-t2955648
Originally it was developed for the Galaxy Note Edge but has been compiled/ported to work with many other devices including the N5. On the N5 it still uses the left lower speaker for audio output but also uses the top earpiece speaker for an audio output as well.
I actually just installed the surround sound mod on my N5 just a couple of mins ago to give it a try and it actually works very well. It has a nice sound balance, haven't noticed any distortion and it is pretty loud... Quite a bit louder than stock. So far it works with my music players, alarms and etc..
I believe V4A and some of the other audio mods have had issues with 5.0 currently and are not working properly on the N5 as of now.
There is a bug list or bug Tracker... You can google it or if I have time I'll post the link for you.
Hope this helps and it definetly might be something worth looking into.
