OK you can use whatever software you like to play audio but if you'd like to test the speaker output, here's some quick commands that will let you verify your amp and speaker are working as they should!
speaker-test -c2 to generate white noise out of the speaker, alternating left and right. Since the I2S amp merges left and right channels, you'll hear continuous white noise
Once you've got something coming out, try to play an audio file with speaker-test (for WAV files, not MP3)
speaker-test -c2 --test=wav -w /usr/share/sounds/alsa/Front_Center.wav
You'll hear audio coming from left and right alternating speakers
If you want to play a stream of music, you can try
sudo apt-get install -y mpg123
At this time, Jessie Raspbery Pi kernel does not support mono audio out of the I2S interface, you can only play stereo, so any mono audio files may need conversion to stereo!
Many programs like PyGame and Sonic Pi have volume control within the application. For other programs you can set the volume using the command line tool called alsamixer. Just type alsamixer in and then use the up/down arrows to set the volume. Press Escape once its set
In Raspbian PIXEL you can set the volume using the menu item control. If it has an X through it, try restarting the Pi (you have to restart twice after install to get PIXEL to recognize the volume control