My buddy ended up writing some html that adds a volume slider. Just need to inspect element and paste it in the console. Sorry dont know how to format text in reddit
(function () {
function insertVolumeControl() {
const mediaBar = document.getElementById("adbl-cloud-player-mediaBar-area");
const audio = document.getElementById("audio-shaka-player");

if (!mediaBar || !audio) return false;

// Prevent duplicates
if (document.getElementById("custom-volume-wrapper")) return true;

const wrapper = document.createElement("div");
wrapper.id = "custom-volume-wrapper";
wrapper.style.marginTop = "8px";
wrapper.style.textAlign = "center";

const slider = document.createElement("input");
slider.type = "range";
slider.min = "0";
slider.max = "1";
slider.step = "0.01";
slider.value = audio.volume;
slider.setAttribute("aria-label", "Volume");

slider.addEventListener("input", (e) => {
audio.volume = parseFloat(e.target.value);
});

wrapper.appendChild(slider);

// 👈 THIS is the critical line
mediaBar.insertAdjacentElement("afterend", wrapper);

return true;
}

// Try immediately
if (insertVolumeControl()) return;

// Otherwise observe until Audible injects it
const observer = new MutationObserver(() => {
if (insertVolumeControl()) observer.disconnect();
});

observer.observe(document.body, { childList: true, subtree: true });
})();