Welcome to the forum.
Tune in a radio station. Check if sound is the same as with the BG, if no, check your cartridge, normaly at this age the cartridge is bad.
If yes, check your S45s driver surrounds, change left to right to see if the quiet one is the same, if not connect a headphone and check for diffences in L/R, if yes, open the BM and check caps, end stage trimmers, bass, treble and balance slide potis.
If you can solder and know a little about electronics I recommend:
BM, change all electrolytic caps, check/change trimmers, check/repair slide potis, change all lamps, adjust no load and endstage currents.
S45s, check/change driver surrounds, change El caps in crossovers, check/change resistors.
BG 1102, clean/check all mecanics (no wear or broken parts), clean all contacts, oil and grease all moving parts and gears, check that all mecanics works properly
I have a BM 2400, BG 2404, S45s, and the above mentioned is what I did, now it all works like a dream.
Beosound 3000, BL 4000, BL 8000, BG 2404,BG 5000, BG CD50, Beocord 5000, BM 901, BM 2400, BM 4000, BV S45, BV 3702. There is nothing we cannot do, but a lot of things we don't want to do!!