The message:
Waiting for available socket...
is shown, because you've reached a limit on the ssl_socket_pool either per Host, Proxy or Group.
Here are the maximum number of HTTP connections which you can make with a Chrome browser:
Maximum per Host: 6 connections.
This is likely hardcoded in the source code of the web browser, so you can't change it.
Total 256 HTTP connections pooled per browser.
Source: Enterprise networking for Chrome devices
The above limits can be checked or flushed at chrome://net-internals/#sockets
(or in real-time at chrome://net-internals/#events&q=type:SOCKET%20is:active
).
Your issue with audio can be related to Chrome bug 162627 where HTML5 audio fails to load and it hits max simultaneous connections per server:proxy. This is still active issue at the time of writing (2016).
Much older issue related to HTML5 video request stay pending, then it's probably related to Issue #234779 which has been fixed 2014. And related to SPDY which can be found in Issue 324653: SPDY issue: waiting for available sockets, but this was already fixed in 2014, so probably it's not related.
Other related issue now marked as duplicate can be found in Issue 401845: Failure to preload audio metadata. Loaded only 6 of 10+ which was related to the problem with the media player code leaving a bunch of paused requests hanging around.
This also may be related to some Chrome adware or antivirus extensions using your sockets in the backgrounds (like Sophos or Kaspersky), so check for Network activity in DevTools.