Ios – WKWebView embed video keeps playing sound after release

I'm implementing a service for playing video files with a webview. I migrated from UIWebView to WKWebView, and trying to play Youtube and Coub videos with it. Almost everything is okay, no crashes in iOS 8, but after WKWebView released and removed from screen, video sound keeps playing for a while (in some cases about 45 seconds).

I can't find the way to stop sound of video. I tried to catch system notifications with a media player, but no success.

Is there any way to stop sound or video in WKWebView?

WKWebview configuration is next:

//javascript for configurate video viewport (not full screen)
NSString *jScript = [NSString stringWithFormat:@"var meta = document.createElement('meta'); meta.setAttribute('name', 'viewport'); meta.setAttribute('content', 'width=%d, height=%d, initial-scale=1, maximum-scale=1'); document.getElementsByTagName('head')[0].appendChild(meta);", (int)VIEW_WIDTH, (int)VIEW_HEIGHT ];

WKUserScript *wkUScript = [[WKUserScript alloc] initWithSource:jScript injectionTime:WKUserScriptInjectionTimeAtDocumentEnd forMainFrameOnly:YES];
WKUserContentController *wkUController = [[WKUserContentController alloc] init];
[wkUController addUserScript:wkUScript];

WKWebViewConfiguration *config    = [WKWebViewConfiguration new];
config.mediaPlaybackAllowsAirPlay = YES;
config.userContentController      = wkUController;

_wkWebView = [[WKWebView alloc] initWithFrame:CGRectMake(0, 0, VIEW_WIDTH, VIEW_HEIGHT) configuration:config];
[_wkWebView setBackgroundColor:RGB(0x0a0a0a)];
[_wkWebView setNavigationDelegate:self];
[self insertSubview:_wkWebView atIndex:0];