Please take a look at the media syncing intro video. If you prefer not to watch a video, please read on.
Recent Anki and AnkiMobile versions synchronize sounds and images in the background, allowing you to continue using the app while they are transferred. Until the media files have been transferred, audio may fail to play, and images may fail to appear if those files are not available yet.
If you tap/click on the sync button when a spinner is shown next to it, it will reveal more details about the syncing progress. If you are waiting for sounds/images to start working, please wait for that window to show "media sync complete". Once that message has been displayed, it indicates all media files have been transferred to or from your device, and the media should then work during study.
If you turn your device's screen off, or you switch to a different app, it will interrupt the sync. When you sync again, the download will resume where it stopped.
If you have a lot of media, the initial sync may take quite a while. AnkiMobile 2.0.64+ will keep the screen turned on while the media syncing screen is shown. Older AnkiMobile versions did not do this, so if you are on an older version and you leave your device unattented, the screen may turn off, interrupting the sync. You can check the version you're running in the Preferences>About screen. If you are on an older version, you can download the free update by searching for AnkiMobile in the app store.
The media sync screen will show increasing counts as progress is made. If you find the added/removed count does not change for an extended period, you can use the Abort button to interrupt the sync, then sync again to pick up where you left off.
If sounds or images fail to appear after "media sync complete" is shown, please check that you have syncing of sounds and images enabled in the preferences screen of each Anki version you use, and please use the "Check Media" function in the preferences screen or tools menu to check if you are missing files.