I can't find a direct route to Voss from Balestrand. The Flam Express Boat webite only has a PDF that shows only 5 or so departure time in the morning.
Use ferry to Vangnes and take taxi or rarely operating bus line 970 (choose Hardanger & Voss in dropdown).
Try the Skyss bus travel planner at https://www.skyss.no/en/timetable-and-maps/travel-planner/ - they are the only direct public transit option.
If you catch a ferry from Balestrand to Flam there is also the option of the NW420 bus from Flam to Voss but it is not very frequent service - see https://www.nor-way.no/en/routes/sognefjordekspressen/
The rome2rio website suggests it can be done by ferry/bus in 2 hrs 34 minutes - but the start time is 7:50am - see https://www.rome2rio.com/map/Balestrand/Voss