For some strange reason, when the Nexus 5 got its Lollipop update, the new stock system sounds were left out. These include updated versions of all of the ringtones and notifications, which Google revamped with a more mature sound that matches Android 5.0's elegant Material Design.
Google's other devices, such as the Nexus 6 and Nexus 9, received the revamped sound files, but Nexus 5 owners were left out in the cold. That is, until the good folks over on XDA put together a flashable ZIP file that will bring all of the new sounds to the Nexus 5.
To download the flashable ZIP that will add the new system sounds, point your Nexus 5's web browser to this link. The download should begin automatically, but wait until it has finished before proceeding to the next step.
Next up, you'll need to boot your phone into custom recovery. To do that, start by powering your Nexus 5 completely off. When the screen goes black, press and hold the volume down and power buttons simultaneously.
This will bring up Android's bootloader menu. From here, press the volume down button twice to highlight "Recovery mode," then press the power button to select it.
Before flashing anything in your custom recovery, it's always a good idea to make sure you have a fresh NANDroid backup on standby. This is particularly useful in this case, since there is no revert ZIP available for this mod. If you end up preferring the old system sounds, the easiest way to get them back would be to restore your backup.
When you're ready to flash the ZIP, go ahead and tap the "Install" button. From here, navigate to your device's Download folder, then select the "Android_L_Sounds.zip" file.
Next, just swipe the slider at the bottom of the screen to install the mod, then when that's finished, tap "Reboot System."
When you get back up, all of your ringtones and notification sounds will be updated to the new Lollipop style. These sound files have the same names and overall feel of the older sounds, they've just been retouched a bit with a more refined flair.