I'm what you hear when you don't hear the words I'm an indistinguishable sounds of a crowd Or an unusual beat in your heart

Once I was worth more than a fortune Men fought and died over me Now I'm only remembered when needed in a pinch

I'm a gateway for singles. Sometimes I have bars, sometimes I look like a type of door. You will usually encounter me at the start and the end. I thrive in subway or metro stations. Can you guess what am I?

Apple, a Don McLean song, or a teen comedy movie

I'm in the process of downsizing. If you remove the charge at the end, we'll have a formal agreement. I'm flexing your muscles. I'm also in every sentence of this riddle.