If your soprano, or any other scale uke for that matter, sounds 'sour' when fretting in frets 1-5 when playing chords, but matches the open strings perfectly on a tuner, your intonation could be off + or -, sharp or flat...
You can check this, e.g., buy making sure the open strings are in tune, and then with the tuner still on, play a G Maj cord, but pluck each string individually, and watch the tuner, and see if they are sharp.
If so, your nut slots are likely too high, or as said above, your are squeezing the strings so tight that they are pressing on the wood of the fretboard, which is wholly unnecessary, as you only need to press down just enough for the string to rest on the leading edge of the fret itself, and should never have to be touching the wood of the fretboard, unless your frets are very shallow above the fretboard.
Correcting intonation at the nut is another topic, which can be explained later if necessary.