Enable @typescript-eslint/explicit-function-return-type in /src (#9788)

* Enable `@typescript-eslint/explicit-member-accessibility` on /src

* Prettier

* Enable `@typescript-eslint/explicit-function-return-type` in /src

* Fix types

* tsc strict fixes

* Delint

* Fix test

* Fix bad merge

src/utils/createVoiceMessageContent.ts

@@ -14,7 +14,7 @@ See the License for the specific language governing permissions and
 limitations under the License.
 */
 
-import { IEncryptedFile, MsgType } from "matrix-js-sdk/src/matrix";
+import { IContent, IEncryptedFile, MsgType } from "matrix-js-sdk/src/matrix";
 
 /**
  * @param {string} mxc MXC URL of the file
@@ -31,7 +31,7 @@ export const createVoiceMessageContent = (
     size: number,
     file?: IEncryptedFile,
     waveform?: number[],
-) => {
+): IContent => {
     return {
         "body": "Voice message",
         //"msgtype": "org.matrix.msc2516.voice",