The idea of generating a random sequence, modifying it and producing "music" with it is not new. Swarms have in fact been suggested for music composition and "Evolutionary Music" is a very actively researched area (e.g. see this link).
So, there might be some background already that you might want to consult before embarking to your own experiments. Now, as far as your questions are concerned:
how to filter a high-dimensional path to a 1d signal that conveys the kind of motion
I wonder if by filter, here, you actually mean, project. You can definitely use an $f(\Theta)$ to combine in an arbitrary way individual components of some set of vectors $\Theta$. In fact, you could even have different $f_n(\Theta)$ to represent different sources of sounds. (e.g. different instruments, or different evolving parameters of the same source). Of course, once a single vector has been produced (at the output of one of those $f$s above), then actually filtering it is just a case of applying some digital filter)
how to convert that to pleasant sounds.
The keyword here is pleasant. And of course ,subsequently we might ask "for whose ears?". Pleasant is subjective. You can achieve Pleasant by simply mapping a sequence to a music scale. In fact, you don't even need complex bee swarm models for this. Just produce numbers from different distributions and then map them on the notes of a scale and send them out to MIDI. That will give you something Pleasant. For example, consider the permisible tones in a C major scale: C D E F G A B. For a given sequence of random integers, do a modulo by 7 and then map them to MIDI messages at some tempo (e.g. eighths at 92 BPM). This will give you a pleasant sounding melody, rather than, what you get when you feed that same sequence of random numbers directly to your speakers (e.g. white noise). Now, to change the "feeling", keep the sequence the same but change the scale that these numbers map to to: C D Eb F G Ab Bb. That's a minor scale. Different intervals between the tones, different end-result.
And this in fact, is a better point of focus than Pleasant. That is, Harmony. And here is a piano to play music in a non-standard harmony, harmony that emerges by dividing the semitone. A different kind of Pleasant.
Hope this helps.