[syhprum (OP)]

By 1928 a proposal was made to lay a single channel speech cable across the Atlantic are any technical details of this proposed project available ?.
The plan was abandoned due to the increasing efficiency of the radio telephone service.
At this time the telegraph cables were achieving a bandwidth of 100 Hz, if modern computer technology had been available could this have been used for speech (a VOCODER was first demonsrated at the worlds fair in 1939) 

[Vern]

I suspect that the repeaters and amplifiers on each end of the cable had more to do with its bandwidth than the cable itself.

This Wiki article about it is intersting
Wildman Whitehouse ruined the first cable a month into its existance.

Cyrus West Field was the force behind the first transatlantic telegraph cable, attempted unsuccessfully in 1857 and completed on August 5, 1858. Although not considered particularly successful or long-lasting, it was the first transatlantic cable project to yield practical results. The first official telegram to pass between two continents was a letter of congratulation from Queen Victoria of the United Kingdom to the President of the United States James Buchanan on August 16. The cable was destroyed the following month when Wildman Whitehouse applied excessive voltage to it while trying to achieve faster telegraph operation. The shortness of the period of use undermined public and investor confidence in the project, and delayed efforts to restore a connection. A next attempt was undertaken in 1865 with much-improved material and, following some setbacks, a connection was completed and put into service on July 28, 1866. This time the connection was more durable, and increased public confidence resulted when the 1865 cable was repaired and put into service shortly afterwards.

[lyner]

I suspect that the repeaters and amplifiers on each end of the cable had more to do with its bandwidth than the cable itself.

The bandwidth limitation will have been due to the construction of the cable;  inductance, capacity  and resistances (series and leakage).

[Vern]

I didn't research the assumption so you're probably correct. I was thinking of the great strides we've made in transmitting information over common telephone wiring.