Is there any Sound enhancers/equalizer?

PulseAudio Equalizer is the way to go.

Here’s a blog post about it: http://www.webupd8.org/2013/10/system-wide-pulseaudio-equalizer.html

Since pulseaudio-equalizer is part of Ubuntu 17.04, you can simply enable the universe repository and then issue the installation command:

sudo apt-get install pulseaudio-equalizer

For older releases, add the PPA first:

sudo add-apt-repository ppa:nilarimogard/webupd8
sudo apt-get update

• Are PPAs safe to add to my system and what are some "red flags" to watch out for?

After PulseAudio Equalizer is installed, you can launch its GUI with the applications qpaeq or pulseaudio-equalizer-gtk.

Psyke83's on the Ubuntu Forums wrote a 'script' to do this for PulseAudio.

Ubuntu 10.10:

Currently the easiest method is to install from a deb created by WebUp8.

Ubuntu 10.04 and below:

There is a PPA containing the equalizer:

sudo add-apt-repository ppa:psyke83/ppa
sudo apt-get update && sudo apt-get install pulseaudio-equalizer