I want to be able to know what volume I'm playing my music in decibels when I'm using my Macbook Air. Is there any plug-ins or methods to show this information?
how far away? is your macbook air connected to speakers?
Your file has a "loudness" (RMS level) -> player volume (gain) -> system volume (gain) -> audio interface voltage specs (gain/level) -> amp (gain) -> speakers (sensitivity). So it's impractical to try and "compute" it with that many variables. Grab an SPL meter.
There are SPL meter smartphone apps that would allow you approximate measurements, but nothing too accurate.
If you're only looking to figure out the attenuation when you press the volume up and down keys, I couldn't find anything with a quick search. Play some pink noise and you can measure by how many dB each "square" of the indicator affects the output level.
The dB level depends on what you are playing and how far away you are from the speakers. The only way to get an accurate estimate of the SPL is to use a SPL meter.
In the box you can use Audio Hijack pro to Monitor your Level in dBFS. As it leaves your speakers you need a SPL Meter. There are some very Rough iphone apps that meassure the spl. And they are a good guideline!