What you already tried to do?

PS: I moved this discussion to the Technical support group, as it was placed in the wrong group

To unregister the activity page and the activity site menu entry add the following two lines to the init function of a suitable plugin:

elgg_unregister_page_handler("activity");
elgg_unregister_menu_item('site', 'activity');

Suitable plugin could be a plugin you use for making custimizations anyway (if in doubt a new plugin with just these two lines in the init function in start.php of this plugin and then placing this plugin at the bottom of the plugin list).

If you use the custom_index plugin (listed as "Front Page demo plugin") you can make the blog page the default landing page by adding the file mod/NAME_OF_YOUR CUSTOMIZATION_PLUGIN/views/default/resources/index.php to your plugin with the content

<?php
forward('blog/all');

This will just forward from the usual index page to the blog listing page. If you use a theme plugin that uses its own index page you might need to override another view instead of this index.php view to replace the landing page provided by this theme plugin.

The activity widgets will still exist and could be added to profile pages and dashboards of site members. I'm not sure right now if there's an unregister function for widgets in Elgg.

As you had asked by PM also about how to remove other menu items my suggestion about that: disable any plugin(s) you don't need or don't want your site members to use. With the plugin disabled the corresponding menu entry will be gone, too. If you only want to remove the menu item do it in the same way as with the activity page entry by using the elgg_unregister_menu_item() function with the item name corresponding to the menu entry you want to remove.

Sounds like there is a problem with the .htaccess

see http://learn.elgg.org/en/stable/intro/install.html#troubleshooting

Please, stop to clone same discussions: 1, 2

Upgrade to Elgg 3, disable Activity plugin and be happy.

Any reason why don't use search here?

https://elgg.org/discussion/view/3001137/new-to-elgg-easy-way-to-change-index-page

https://elgg.org/discussion/view/2990012/how-to-remove-activity-page

https://elgg.org/discussion/view/2973191/how-to-remove-all-activity-page

I recently wrote a blog about how to create a multimedia site with elgg. I used the files plugin, nubeso player, among others. You can read about it here. It could be a good starting point.

I would suggest to use the Files plugin. It should support mp3 without any changes and should also allow to play them on site without any additional changes necessary.

If you don't have the skills to modify the community_plugins plugin according to your needs forget about it. It's not just one place in the code that might need to be changed but a number of changes (even if each one is small) is probably needed to get everything working according to your needs. And I don't expect you will find anyone who would spend hours for free to guide you here.

Also the community plugins isn't the best example of good Elgg coding standards ;)

As others have mentioned try the Files plugin first. It's better written and easier to adopt to your needs