Using the latest pimusicbox with an old rbp 1b. I love this project! The first 10 seconds of each new song on Spotify start with very bad sound quality, crackling and more noises. Does this raspberry not have enough ram maybe?
My first guess would be that it’s more a cpu issue. During the first few seconds of playback the default webclient does a lot of requests to mopidy for the track playback state and position. Could you try one of the other bundled clients (http://musicbox/moped) or an MPD client to see if that is any different. The behaviour there should be consistent regardless of music source (tunein, Spotify, YouTube, local etc).
One thing that is unique to Spotify is that during the first few seconds it tries to download the whole song (so you can seek instantaneously). Trying a different music source of similar quality and codec would be a good experiment to try here, a 320kbps Ogg local music file would be best. We previously found that this effect was much worse with Spotify caching enabled, so we’ve had it disabled for a long time now. I think all this means is that the downloaded file is not saved to the SD card as it would usually be.
I admit I don’t really test much with the old model pi any more.