The bard introduces a powerful song into his defenses which works with the vibrational energies of the area using the sonic intensity and magic of the troubadoor's training/skill level. The song focuses on area effect spells as well as maneuvers most effectively but carries a small chance to be effective against any oncoming attack. Imagine the bard is singing the song in a way which touches every resonant frequency of the living and breathing world surrounding them. The attunement to the flux of environs and of the critters within it affords the bard a 'squeezing' effect so that the distance of what the physical world is comprised of appears more distant and smaller, as if set up on a plane for the bard to assess all at once rather than one individual object or critter at a time. They are able to gain the advantage by perceiving threats earlier but more importantly of introducing weavings of musical magic into the oncoming aggressions in such a way which can reduce their harm or remove them as a harm altogether. For example, a Grimswarm Ranger casts a Tangleweed which thrashes about and attempts to wrap/pull the bard down. The Song of Compression makes the object appear much more minute for the instance of the attack so the bard can sort of have a flash of
decision making and chance to use their song creatively to combat the assault. The weed might miss altogether, disappear at best or have a far less damaging effect if it does work. If it's a Rogue trick coming from a critter, then the bard has a quicker reflex time and evaluation process for the incoming maneuver. It's like listening and connecting with the environment using sound as a paintbrush, in a way. In my mind, I see it as having a 5% chance without extra lore to work on a normal physical or bolt attack. When it comes to maneuvers, the odds will be the highest as they are coming directly from an organism with a very broad signal to the bard. As for gas clouds, tangleweeds, other area effect spells which linger around they can trigger on a 50/50 basis. I like to imagine it can trigger a LOT (Wall of Thorns is effective but overall, it's disappointing not to see it kick in a lot more or with a bit more capability..as an example). But it could have a totally different rate of success, like a dROLL could determine the overall effectiveness-- say a '1' gives you a momentary +10 bonus to your applied defense against a manuever and a dROLL 100 would totally eliminate the threat, leave you with a clean shot against them. If it's an e-wave, the odds will be a lot higher since it's in the bards spell knowledge base, an elemental spell easily detectable. Tangleweeds would have very low odds.
Clouds can be eliminated entirely by the song but it might be a question of level and skill, a real challenge for like-level critters (I really like the 50/50 mindset here).

Also I would like to see an automatic +10 to Perception and (drumroll....) specifically to Treasure.

I think Song of Loot deserves a whole new post though.