In his book, Floyd Toole talks a lot about "early reflections" (mostly desirable) and I have one quote specifically about sidewall reflections:
"In uncontrolled experiments with audiophile friends, we decided that absorbing side wall (sound) reflections seemed to flatter some recordings (mostly pop/rock) while leaving the walls reflective flattered others (mostly classical and jazz)."
"The treatment of early reflections boundary areas is "optional": reflect, diffuse, or absorb, as the customer prefers."
"In the field architectural acoustics it has long been recognized that early reflections improve speech intelligibility."
"Early (sound) reflections had the same desirable effect on speech intelligibility as increasing the level of the direct sound... Late reflections (including reverberation) are undesirable, but reducing them should not be the first priority, which is to maximize the total energy in the direct and early-reflected speech sounds. Remarkably, even attenuation the direct sound had little effect on intelligibility in a sound field with sufficient early reflections."
"For speech intelligibility - a crucial consideration for movies - it is early reflections that are the main contributors. Early refection energy arriving within about the first 50 ms following the direct sound has the same effect on speech intelligibility scores as an equal increase in the direct sound energy."
"Early reflections also reduce the thresholds for detecting resonances in loudspeakers, making flawed loudspeakers more noticeable and revealing more timbral subtleties in music."
"Early reflections can reduce the timbral degradation and speech intelligibility loss in the phantom center image."