Juicebox-Pro supports only one audio track so if you want a whole album to be played, you'd need to merge all the tracks into one.

Having said that, the audio should start playing as soon as the Audio Button is clicked, without the need to wait for the entire mp3 file to be downloaded in its entirety first.
I've just checked this in a test gallery on my own web server using with an 83MB mp3 file in a couple of browsers (Edge and Firefox) and it works fine.

It should also be noted that, due to recent browser restrictions, it is no longer possible to automatically start the audio playing as soon as the gallery loads in all browsers.
When Juicebox-Pro was first released, it was possible to autoplay audio on loading a gallery by setting playAudioOnLoad="TRUE" (in the Audio Options).
However, fairly recently, certain desktop browsers (including Chrome and Firefox) have started to block the autoplay of audio (and video) by default.
Users now need to initiate the playing of audio (and video) files themselves.
In the case of a Juicebox-Pro gallery, users now need to click the Audio Button (on the gallery's Button Bar) to start audio playing.
I have just checked my own installed browsers a few minutes ago and playAudioOnLoad continues to work only in Microsoft Edge and Opera.

Here are the release notes for Firefox 66 where this change of behavior was first noted: https://www.mozilla.org/en-US/firefox/6 … easenotes/
Here are similar notes for Chrome: https://developers.google.com/web/updat … cy-changes

As unfortunate as it is, there is nothing that can be done from within Juicebox-Pro to force audio to autoplay in browsers which feature these audio/video restrictions.
Users will likely have the option of changing their browser's settings (depending on the browser) to allow autoplay of audio (and video) but this is something that the user must do. (A user's browser settings cannot be changed from within a Juicebox gallery.)

However, as long as you display the Audio Button on your gallery's Button Bar (by setting showAudioButton="TRUE"), then users can start the audio track themselves by clicking the button.