unfortunately it's currently only possible to link 1 file to be played as the background sound via the properties tab, it's a shame Simon can't make it so we can insert multiple sounds to be looped. Actually, he could probably sort that out - will ask him later if it's possible to implement for a future update - but for now, yeah you guessed correctly, just start sounds playing inside of an at begin of scene action or inside of a called by other action you called inside of the at begin of scene action.
In regards to dynamic sounds, if it's not a loop & you want the sounds to play every so often randomly then create a value (1 per sound) & a called by other action block (1 per sound again). Now what you can do is make the action block loop & using the value you created you can define a random delay each time it loops so that the sound will play dynamically every so often, you could even make it loop the sound a random amount of times too if you wanted to...
Right, now here's a quick breakdown of what's going on in the screenshots. As you can see I have created an at begin of scene action & a called by other action which I have renamed to snd_bird_(1). In the at begin of scene action I am calling the snd_bird_(1) action. Now in the snd_bird_(1) action block I have set 2 the 2 values I have created in the scene values column as random values. One controls the delay between each loop of the action block & the other specifies the amount of times we should loop the sound in a row. If the loop value is more than zero we loop back to the action part #3 which is the query that checks if loop is more than zero & if it is then it plays the sound again & if it isn't then it loops back to the beginning of the action block. You may have noticed that I created a pause action part after setting the 2 random values. I have linked the delay value that we created to it so that it pauses for the time specified in that - in our case anywhere between 1000ms (1 second) & 10000ms (10 seconds).
Sorry if this explanation doesn't make much sense. Anyway, you can use these sort of looping action blocks for so many different things. You could dynamically determine when to play an animation that has a bird or flock of birds flying across the scene, or when a cricket should jump, or a tree should rustle & so on & so on. It's just a really useful method for dynamically controlling things in your scenes.