NASA - Apollo 15 Radio - 52 Thousand Miles From Earth - Tape 543/544
The Apollo 15 crew of David Scott, Alfred Worden, and James Irwin were en route to the Moon after the Trans-Lunar Injection burn.
This is a recording, thats been slightly modified and cleaned up. Its from Tape 543 and 544
Here is a brief summary of events that happened that day -
After TLI - The Translunar Coast (where tape 543 fits)
Once the TLI burn was complete, the crew's velocity had built to around 35,000 feet per second at cut-off, though this was already dropping as they climbed away from Earth. The estimated time of their closest approach to the Moon was 78 hours, 34 minutes into the mission. The task immediately ahead was separating the Command Service Module from the S-IVB stage, docking with the Lunar Module Falcon, and extracting it — in what was, up to that point, shaping up to be a completely nominal mission. NASA [https://www.nasa.gov/history/afj/ap15fj/03tde.html]
Transposition, Docking & Extraction
After TLI, the spent S-IVB stage needed careful management — its propellant tanks, though well insulated, were only designed to last through boost and a few hours past injection, and the building internal pressures were vented regularly to prevent tank rupture. The venting was done through opposing openings to avoid imparting unwanted rotation on the stage. In about three days' time, the spent booster would impact the Moon at 1°31'S, 11°49'W. NASA [https://www.nasa.gov/history/afj/ap15fj/03tde.html]
Early Mission Drama - SPS Troubleshooting
One of the most dramatic early events, covered in its own journal chapter, was a problem with the Service Propulsion System (SPS) engine - the critical engine needed both to enter lunar orbit and to return home. Mission Control spent considerable time troubleshooting it before giving the crew a "go" for a normal mission.
Tape 543 in context
Given that the tape collection starts at 540 and the TLI burn happens around the 2-hour mark of the mission, tape 543 most likely covers the very first hours of the translunar coast — the three-day journey to the Moon. At this stage the crew would have been performing the docking and extraction of the Lunar Module, running spacecraft systems checks, and beginning the slow, quiet coast towards the Moon while Mission Control monitored everything from Houston.