I didn't test, but I think something like the following patch could fix the issue:

diff --git a/app/views/audit_mailer/summary.html.erb b/app/views/audit_mailer/summary.html.erb
index f1b490df9..fb6392840 100644
--- a/app/views/audit_mailer/summary.html.erb
+++ b/app/views/audit_mailer/summary.html.erb
@@ -11,10 +11,10 @@
           <%= link_to(audit.username, audits_url(:search => user_search_param)) %>
           <span style="color: #cccccc"><%= audit_remote_address audit %></span>
           <%= audit_action_name audit%> <%= audited_type audit %>:
-          <%= link_to(audit_title(audit), audit_url(:id => audit), {:style => 'color: #2a6496'}) %>
+          <%= link_to(audit_title(audit), audits_url(search: "id=#{audit.id}"), {:style => 'color: #2a6496'}) %>
         </b>
         <ul style="list-style-type: none;">
-          <% details(audit, audit_url(audit)).each do |detail| -%>
+          <% details(audit, audits_url(search: "id=#{audit.id}")).each do |detail| -%>
             <li>
               <%= detail %>
             </li>
diff --git a/app/views/audit_mailer/summary.text.erb b/app/views/audit_mailer/summary.text.erb
index f0862265f..8e341b76b 100644
--- a/app/views/audit_mailer/summary.text.erb
+++ b/app/views/audit_mailer/summary.text.erb
@@ -5,7 +5,7 @@
          'Displaying %{num_audits} of %{total_audits} audits', @count) % {:num_audits => @limit, :total_audits => @count} %>
   <% @audits.each do |audit| %>
     <%= audit.username %> (<%= audit.remote_address %>) <%= audit_action_name audit%> <%= audited_type audit %>: <%= audit_title(audit) %>
-    (<%= audit_url(:id => audit) %>)
+    (<%= audits_url(search: "id=#{audit}") %>)
   <% end %>
 <% else %>
   <%= _('No audit changes for this period') %>
diff --git a/config/routes.rb b/config/routes.rb
index b80a4edbf..75ace534e 100644
--- a/config/routes.rb
+++ b/config/routes.rb
@@ -264,7 +264,7 @@ Foreman::Application.routes.draw do
     end
   end

-  resources :audits do
+  resources :audits, :only => [:index] do
     collection do
       get 'auto_complete_search'
     end

it would be nice to expand the list if there is only one match to avoid the extra click

for reference, the error that is shown on the UI when trying to follow the link:

Oops, we're sorry but something went wrong AuditsController#show is missing a template for this request format and variant. request.formats: ["text/html"] request.variant: [] NOTE! For XHR/Ajax or API requests, this action would normally respond with 204 No Content: an empty white screen. Since you're loading it in a web browser, we assume that you expected to actually render a template, not nothing, so we're showing an error to be extra-clear. If you expect 204 No Content, carry on. That's what you'll get from an XHR or API request. Give it a shot.