I think you are on the right track in your own comments in the sample code. You have to convert your array to an array of numbers first. Not just to do the sum, but before you do the sort, or you will be sorting in alpha order, not numeric order.
Something like:
let strArray = [activitiesOfDailyLiving, socialRecreational, travel, interpersonalRelationships, concentrationPersistencePace,resilienceEmployability];
// convert strings to numbers in a separate array
let numArray = strArray.map(n => Number(n));
// sort (default numeric sort will work)
numArray = numArray.sort();
// sum
let sum = numArray[2] + numArray[3];