Speakers before 1870?

The first electronic amplifiers were designed and marketed in the 1920s, so any history of loudspeakers also starts there.

I would research such companies as Western Electric, Bell Laboratories, RCA, Edison, Tannoy and Wharfedale, who were pioneers in the industry.

Bell laboratories did extensive acoustical reserch in the era around 1900 to determine the audibility of early telephone microphones and receivers, which were the earliest thing that could be considered a "speaker" or "earphone" (but not a loudspeaker).

Fletcher and Munson, at Bell, developed the "Fletcher-Munson Curves", which describes the sensitivity of the human ear to various frequencies at different loudness levels with a family of graphs.

in the present and near future, "powered" or "active" speakers are becoming more common and popular.  These have amplifiers built into the speaker cabinet, and the convenience/compactness/visual appeal is attractive to many people.  Now that more people are streaming music into multiple areas of the home, it's appealing to just have speakers placed in the various rooms rather than a receiver/amplifier plus speakers.  

(I like seeing the amplifiers & such, but it's not about me)

Wireless of every variation is becoming more popular every minute.  The next step I think will be speakers that don't have speaker cables attached - that is, music will be streamed to wireless receivers/players inside the speakers.

One thing I hope for, and do not necessarily predict, is that a concern for high quality/high fidelity will become more widespread again.  With the downsizing and convenience that has been emphasized in the mass market over the past decade or two, I think many people are listening to sound that is actually poorer in quality than the old systems they used to have.  

The much greater prevalence of headphones/personal audio will help with this.  Better bang for the buck, compared with full size stereos, if one chooses headphones wisely.  Although they can never fully supplant music coming through the air at you, in a room.